requestOptions precondition check polishing

src/objective-c/GRPCClient/GRPCCall.m

@@ -98,7 +98,7 @@ const char *kCFStreamVarName = "grpc_cfstream";
 - (instancetype)initWithRequestOptions:(GRPCRequestOptions *)requestOptions
                        responseHandler:(id<GRPCResponseHandler>)responseHandler
                            callOptions:(GRPCCallOptions *)callOptions {
-  if (!requestOptions || !requestOptions.host || !requestOptions.path) {
+  if (requestOptions.host.length == 0 || requestOptions.path.length == 0) {
     [NSException raise:NSInvalidArgumentException format:@"Neither host nor path can be nil."];
   }