RC: fix internal API nullability issue caught by analyzer (#13712)

Author: paulb777

FirebaseRemoteConfig/Sources/Private/RCNConfigFetch.h

@@ -53,7 +53,8 @@ typedef void (^RCNConfigFetchCompletion)(FIRRemoteConfigFetchStatus status,
 /// @param expirationDuration  Expiration duration, in seconds.
 /// @param completionHandler   Callback handler.
 - (void)fetchConfigWithExpirationDuration:(NSTimeInterval)expirationDuration
-                        completionHandler:(FIRRemoteConfigFetchCompletion)completionHandler;
+                        completionHandler:
+                            (_Nullable FIRRemoteConfigFetchCompletion)completionHandler;
 
 /// Fetches config data immediately, keyed by namespace. Completion block will be called on the main
 /// queue.

FirebaseRemoteConfig/Sources/RCNConfigFetch.m

@@ -131,7 +131,8 @@ - (void)dealloc {
 #pragma mark - Fetch Config API
 
 - (void)fetchConfigWithExpirationDuration:(NSTimeInterval)expirationDuration
-                        completionHandler:(FIRRemoteConfigFetchCompletion)completionHandler {
+                        completionHandler:
+                            (_Nullable FIRRemoteConfigFetchCompletion)completionHandler {
   // Note: We expect the googleAppID to always be available.
   BOOL hasDeviceContextChanged =
       FIRRemoteConfigHasDeviceContextChanged(_settings.deviceContext, _options.googleAppID);