call callback handler only when it's not null. (#3057)

Author: Yue-Wang-Google

Firebase/Auth/Source/Auth/FIRAuth.m

@@ -838,7 +838,9 @@ - (void)internalSignInAndRetrieveDataWithCredential:(FIRAuthCredential *)credent
           FIRAuthDataResult *result = user ?
               [[FIRAuthDataResult alloc] initWithUser:user
                                    additionalUserInfo:additionalUserInfo] : nil;
-          callback(result, error);
+          if (callback) {
+            callback(result, error);
+          }
         }];
       }
     }];