Quash -Wmove triggers

Fix code that triggers -Wmove, which isn’t an error today but will be soon. See the referenced bug for full context.

Tested:
    TAP --sample ran all affected tests and none failed
    http://test/OCL:267146993:BASE:267126309:1567606504088:33870910
PiperOrigin-RevId: 267177351

Author: Googler

functions/src/ios/callable_reference_ios.mm

@@ -127,7 +127,7 @@ Error NSErrorToErrorCode(NSError* error) {
 
 Future<HttpsCallableResult> HttpsCallableReferenceInternal::Call() {
   ReferenceCountedFutureImpl* future_impl = future();
-  HttpsCallableResult null_result(std::move(Variant::Null()));
+  HttpsCallableResult null_result(Variant::Null());
   SafeFutureHandle<HttpsCallableResult> handle =
       future_impl->SafeAlloc(kCallableReferenceFnCall, null_result);
   void (^completion)(FIRHTTPSCallableResult* _Nullable, NSError* _Nullable) =
@@ -143,7 +143,7 @@ Error NSErrorToErrorCode(NSError* error) {
   id data = util::VariantToId(variant);
 
   ReferenceCountedFutureImpl* future_impl = future();
-  HttpsCallableResult null_result(std::move(Variant::Null()));
+  HttpsCallableResult null_result(Variant::Null());
   SafeFutureHandle<HttpsCallableResult> handle =
       future_impl->SafeAlloc(kCallableReferenceFnCall, null_result);
   void (^completion)(FIRHTTPSCallableResult* _Nullable, NSError* _Nullable) =