Make codegen'd TurboModule event emitters no-op when the emitter callback is absent

[christophpurrer]

Summary:
The codegen'd TurboModule emitOn<Event> methods invoked their EventEmitterCallback without checking it was set. That callback is installed by the generated *SpecJSI constructor, which runs when JS first looks the module up — so a native module that emits before that point invoked an empty std::function on iOS (std::bad_function_call) or a null field on Android (NPE). Native code commonly holds the module instance and pushes events well before any JS surface mounts, so call sites had to wrap every emit in a try/catch to stay crash-free.

Both generators now read the callback into a local and no-op when it is absent:

• ObjC++ (serializeEventEmitter.js): copies _eventEmitterCallback, calls it only if non-empty.
• Java (GenerateModuleJavaSpec.js): copies the Nullable CxxCallbackImpl field and null-checks it.

The local is for readability, not synchronization: the callback is installed once and never cleared, and Java reference reads are already atomic.

The setEventEmitterCallback lambdas had a separate lifetime bug: they captured eventEmitterMap_ by reference and looked events up with operator[]. The Java/ObjC module owns the callback and can outlive the C++ *SpecJSI that installed it, so a stale callback dereferenced a dangling map; and operator[] silently default-inserted a null shared_ptr for an unknown event name, which the next line dereferenced. They now capture a copy of the map and use a checked find (serializeModule.js, JavaTurboModule.cpp). Every emitter is registered before the callback is installed, so the copy is complete.

Note the scope of the guarantee: it covers the ObjC++ and Java generators, which route through an EventEmitterCallback. A C++-only TurboModule (<Module>CxxSpec, from GenerateModuleH.js) has no callback to check — it emits through eventEmitterMap_ entries its own constructor registers — so the "emit unconditionally" guidance is about the callback, not about emitting before construction finishes.

Changelog:
[General][Fixed] - TurboModule event emitters no longer throw when an event is emitted before the emitter callback is installed

Differential Revision: D115130767

[meta-codesync]

@christophpurrer has exported this pull request. If you are a Meta employee, you can view the originating Diff in D115130767.