fix(android): crash on resetting borderStyle with props 2.0 (#56228)

Summary:
### The problem

When enabling props 2.0 with the feature flags:

```
  override fun enableAccumulatedUpdatesInRawPropsAndroid(): Boolean = true
  override fun enableExclusivePropsUpdateAndroid(): Boolean = true
  override fun enablePropsUpdateReconciliationAndroid(): Boolean = true
```

setting the style for `borderStyle` to undefined causes a crash:

https://github.com/user-attachments/assets/f54fa3cb-44a2-4e35-a542-1013132fc076

<details>
<summary>Expand for crash stack</summary>

```
Error while updating prop borderStyle
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.String
	at com.facebook.react.uimanager.ViewManagersPropertyCache$StringPropSetter.getValueOrDefault(ViewManagersPropertyCache.java:291)
	at com.facebook.react.uimanager.ViewManagersPropertyCache$PropSetter.updateViewProp(ViewManagersPropertyCache.java:86)
	at com.facebook.react.uimanager.ViewManagerPropertyUpdater$FallbackViewManagerSetter.setProperty(ViewManagerPropertyUpdater.kt:155)
	at com.facebook.react.uimanager.ViewManagerPropertyUpdater$GenericViewManagerDelegate.setProperty(ViewManagerPropertyUpdater.kt:188)
	at com.facebook.react.uimanager.ViewManager.updateProperties(ViewManager.java:102)
	at com.facebook.react.fabric.mounting.SurfaceMountingManager.updateProps(SurfaceMountingManager.kt:635)
	at com.facebook.react.fabric.mounting.SurfaceMountingManager.updateProps(SurfaceMountingManager.kt:598)
	at com.facebook.react.fabric.mounting.mountitems.IntBufferBatchMountItem.execute(IntBufferBatchMountItem.kt:127)
	at com.facebook.react.fabric.mounting.MountItemDispatcher.executeOrEnqueue(MountItemDispatcher.kt:379)
	at com.facebook.react.fabric.mounting.MountItemDispatcher.dispatchMountItems$lambda$7$lambda$6(MountItemDispatcher.kt:269)
	at com.facebook.react.fabric.mounting.MountItemDispatcher.$r8$lambda$4xp_oXLXKRRjXh_WfqmwhGWbzaM(Unknown Source:0)
	at com.facebook.react.fabric.mounting.MountItemDispatcher$$ExternalSyntheticLambda4.invoke(D8$$SyntheticClass:0)
	at com.facebook.react.internal.tracing.PerformanceTracer.trace(PerformanceTracer.kt:46)
	at com.facebook.react.internal.tracing.PerformanceTracer.trace(PerformanceTracer.kt:35)
	at com.facebook.react.fabric.mounting.MountItemDispatcher.dispatchMountItems(MountItemDispatcher.kt:250)
	at com.facebook.react.fabric.mounting.MountItemDispatcher.tryDispatchMountItems(MountItemDispatcher.kt:94)
	at com.facebook.react.fabric.FabricUIManager$DispatchUIFrameCallback.doFrameGuarded(FabricUIManager.java:1581)
	at com.facebook.react.uimanager.GuardedFrameCallback.doFrame(GuardedFrameCallback.kt:42)
	at com.facebook.react.modules.core.ReactChoreographer.frameCallback$lambda$1(ReactChoreographer.kt:58)
	at com.facebook.react.modules.core.ReactChoreographer.$r8$lambda$nSkFhrr5T7rop_XKwzlLov4NLLw(Unknown Source:0)
	at com.facebook.react.modules.core.ReactChoreographer$$ExternalSyntheticLambda0.doFrame(D8$$SyntheticClass:0)
	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1628)
	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1639)
	at android.view.Choreographer.doCallbacks(Choreographer.java:1235)
	at android.view.Choreographer.doFrame(Choreographer.java:1160)
	at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1613)
	at android.os.Handler.handleCallback(Handler.java:1070)
	at android.os.Handler.dispatchMessage(Handler.java:125)
	at android.os.Looper.dispatchMessage(Looper.java:333)
	at android.os.Looper.loopOnce(Looper.java:263)
	at android.os.Looper.loop(Looper.java:367)
	at android.app.ActivityThread.main(ActivityThread.java:9287)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:566)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:929)
```

</details>

### The bug & its fix

The problem is that we clear the value of `borderStyle` using `NULL`:

https://github.com/facebook/react-native/blob/4f908f433044279b7340c1933792b15aa5796a32/packages/react-native/ReactCommon/react/renderer/components/view/platform/android/react/renderer/components/view/HostPlatformViewProps.cpp#L382

where `NULL` seems to be defined as `0`:

<img width="545" height="315" alt="Screenshot 2026-03-26 at 09 12 56" src="https://github.com/user-attachments/assets/704ccd04-59f1-453f-8104-f56e57410b2d" />

so, folly treats this as a `0.0` and we get a double for a value meant to be `null` / optional.

The fix is to change this to a proper folly nullish-value, as this PR does. This fixes the crash:

https://github.com/user-attachments/assets/32ab68bc-536d-47be-88e1-74d141e7718b

(I think there are other places where this is being misused, but i will make individual PRs for those cases!)

## Changelog:

<!-- Help reviewers and the release process by writing your own changelog entry.

Pick one each for the category and type tags:

[ANDROID|GENERAL|IOS|INTERNAL] [BREAKING|ADDED|CHANGED|DEPRECATED|REMOVED|FIXED|SECURITY] - Message

For more details, see:
https://reactnative.dev/contributing/changelogs-in-pull-requests
-->

[ANDROID] [FIXED] - Fixed crash when enabling props 2.0 and setting `borderStyle` to `undefined`

Pull Request resolved: https://github.com/facebook/react-native/pull/56228

Test Plan:
You can test the crash before with the following code in the playground app:

https://github.com/facebook/react-native/compare/main...hannojg:react-native:hannojg/reproduction-borderstyle-issue?expand=1

You can then test and see how this fix resolves the crash.

Reviewed By: javache

Differential Revision: D98322741

Pulled By: zeyap

fbshipit-source-id: d80c490b8570bb1a098d6234df91a9d5cade4df3