Correctly handle null values in dynamic props in prop forwarding

Summary:
Changelog: [internal]

shergin found that folly's merge_patch implementation doesn't propagate `null` correctly (details in D26435620 (https://github.com/facebook/react-native/commit/1e9f63fe277c42d812ef007ced7eff1688602b62)). This is a requirement and needs to be adjusted in props forwarding on Android.

As far as we know this isn't causing any bugs but it is an error that should be fixed.

Reviewed By: shergin

Differential Revision: D26545821

fbshipit-source-id: 9edd24aecfcde17f5d9c1197f65db0e0f3f9e364