[RN][Codegen] Better support filtering out non linked platforms

[cipolleschi]

Summary:

As pointed out by this comment, the property dependency.platforms might not be defined, as the format

{
   "dependencies": {
     "dependency-name": {
       "root": "path/to/dependency",
     }
   }
 }

is also supported.
In this case, we assume that the library is linked to all platforms.
We don't consider the case were dependency-name.root is equal to null, because that means that the library is not linked to the app at all, and in that case the dependency should be removed by the user.

Changelog:

[General][Fixed] - Improve support for filtering out non supported platforms

Test Plan:

Tested on an app with this configuration and it does not crashes.