Assert TurboModuleRegistry is not null

Summary:
Looking at the crash reports from T46487253:
1. This crash happens only with TurboModule-compatible NativeModules.
2. Users who experience this crash are in the TurboModules test group.

Therefore, the crash happens while trying to load TurboModules.

The stack trace of the crash includes [this lookup via the NativeModule system](https://fburl.com/diffusion/vxj9goz5). When TurboModules are enabled, we can only start executing this line if one of two things are true:
1. The TurboModuleRegistry is null in CatalystInstanceImpl.
2. The TurboModuleRegistry isn't null but the NativeModule returned by the TurboModuleRegistry is null.

We can protect against 1 by asserting that when `ReactFeatureFlags.useTurboModules` is `true`, `mTurboModuleRegistry` is not null. Once this check lands, unless there's a race with setting `ReactFeatureFlags.useTurboModules`, we should be able to rule out 1.

Changelog:
[Added][Android] - Assert TurboModuleRegistry isn't null before using it in CatalystInstanceImpl

Reviewed By: PeteTheHeat

Differential Revision: D18211935

fbshipit-source-id: de88c033425c474ef80b73386b7182b1d3bb382f