Update FIRAuthInterop to use _Nullable_result annotation

[andrewheard]

The getTokenForcingRefresh:withCallback: method in FirebaseAuthInterop can invoke the completion handler with nil, nil but (void (^)(SomeType *_Nullable value, NSError *_Nullable error)) completion blocks are translated to async throws -> SomeType in Swift async/await. The _Nullable_result annotation, instead of just _Nullable, signals that the return type may be nil even when no error is thrown; see Swift SE-0297 for more details.

Before:
func getToken(forcingRefresh forceRefresh: Bool) async throws -> String

After:
func getToken(forcingRefresh forceRefresh: Bool) async throws -> String?

#no-changelog

[ncooke3]

TIL! Nice find here.

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseDatabase-iOS-FirebaseDatabase.framework

  Overall coverage changed from 56.83% (c9d013b) to 56.78% (383073d) by -0.05%.

  Filename	Base (c9d013b)	Merge (383073d)	Diff
  FIRRetryHelper.m	83.33%	88.10%	+4.76%
  FSRWebSocket.m	38.29%	38.14%	-0.15%
  FWebSocketConnection.m	41.74%	37.19%	-4.55%

• FirebaseFirestore-iOS-FirebaseFirestoreInternal.framework

  Overall coverage changed from 88.21% (c9d013b) to 88.11% (383073d) by -0.10%.

  Filename	Base (c9d013b)	Merge (383073d)	Diff
  exception.cc	84.21%	23.68%	-60.53%
  grpc_stream.cc	98.02%	99.01%	+0.99%
  leveldb_key.cc	98.63%	98.82%	+0.20%
  ordered_code.cc	94.39%	93.90%	-0.49%
  serializer.cc	91.31%	91.00%	-0.31%
  watch_stream.cc	95.35%	90.70%	-4.65%

Test Logs

• Base (c9d013b): https://github.com/firebase/firebase-ios-sdk/actions/runs/8806021050
• Merge (383073d): https://github.com/firebase/firebase-ios-sdk/actions/runs/8807084918

1. https://storage.googleapis.com/firebase-sdk-metric-reports/HHzQRw7pE4.html