Use main queue when dispatch queue is not specified

src/objective-c/ProtoRPC/ProtoRPC.h

@@ -84,7 +84,7 @@ NS_ASSUME_NONNULL_BEGIN
  * should be issued.
  */
 - (nullable instancetype)initWithResponseHandler:(void (^)(GPBMessage *, NSError *))handler
-                           responseDispatchQueue:(dispatch_queue_t)responseDispatchQueue;
+                           responseDispatchQueue:(nullable dispatch_queue_t)responseDispatchQueue;
 
 /** Response headers received during the call. */
 @property(readonly, nullable) NSDictionary *responseHeaders;

src/objective-c/ProtoRPC/ProtoRPC.m

@@ -38,7 +38,11 @@
                            responseDispatchQueue:(dispatch_queue_t)dispatchQueue {
   if ((self = [super init])) {
     _responseHandler = handler;
+    if (dispatchQueue == nil) {
       _responseDispatchQueue = dispatchQueue;
+    } else {
+      _responseDispatchQueue = dispatch_get_main_queue();
+    }
   }
   return self;
 }