Guard against nil methodQueue in RCTBlobManager

Summary:
## Description
In T63516227, we're seeing a crash that occurs because `networking.methodQueue` is `nil`, and we try to `dispatch_async` to it.

## Hypothesis
This looks like a problem with NativeModule cleanup:
1. Some JS executes a call to `RCTBlobManager.addNetworkingHander`. This schedules an async method call on the `RCTBlobManager` method queue.
2. In `RCTCxxBridge invalidate`, on the JS thread, we loop through all the `RCTModuleData`s, and invalidate them. This invalidates our NativeModules (perhaps not all but only `RCTNetworking`).
3. The `RCTBlobManager.addNetworkingHander` method call finally executes, with `RCTNetworking`'s methodQueue set to nil, which throws this error.

Changelog:
[iOS][Fixed] - Fix RCTBlobManager cleanup crash

Reviewed By: PeteTheHeat

Differential Revision: D20498096

fbshipit-source-id: d2d60984637ddf883278289258aa9b2ae81bb172

Libraries/Blob/RCTBlobManager.mm

@@ -142,6 +142,12 @@ RCT_EXPORT_METHOD(addNetworkingHandler)
 {
   RCTNetworking *const networking = _bridge ? _bridge.networking : [_turboModuleLookupDelegate moduleForName:"RCTNetworking"];
 
+  // TODO(T63516227): Why can methodQueue be nil here? 
+  // We don't want to do anything when methodQueue is nil.
+  if (!networking.methodQueue) {
+    return;
+  }
+
   dispatch_async(networking.methodQueue, ^{
     [networking addRequestHandler:self];
     [networking addResponseHandler:self];