Fix crash when error.userInfo[FIRFunctionsErrorDetailsKey] is nil

* crash when not specified `details` parameter at server's `functions.https.HttpsError`

Author: ko2ic

packages/cloud_functions/ios/Classes/CloudFunctionsPlugin.m

@@ -49,6 +49,7 @@ - (void)handleMethodCall:(FlutterMethodCall *)call result:(FlutterResult)result
                                           @"code" : [self mapFunctionsErrorCodes:error.code],
                                           @"message" : error.localizedDescription,
                                           @"details" : error.userInfo[FIRFunctionsErrorDetailsKey]
+                                              ?: [NSNull null]
                                         }];
               } else {
                 flutterError =