Fix blob response parsing for empty body on iOS

Summary:
We currently handle empty body poorly in the iOS blob implementation, this happens because of an early return that cause the blob response to not be processed by the blob module, resulting in an empty string as the body instead of a blob object. We also need to make sure to create an empty blob object when data is nil (empty body) as per the XMLHttpRequest spec. The Android implementation was already handling this properly.

Fixes facebook#18223

Send a HEAD request

```js
fetch('https://apipre.monkimun.com/whoami', {
  body: null,
  method: 'HEAD',
  headers: {
    Accept: 'application/json',
    'Content-Type': 'application/json',
  },
})
```

[IOS][BUGFIX][Blob] - Fix blob response parsing for empty body
Closes facebook#18547

Differential Revision: D7415950

Pulled By: hramos

fbshipit-source-id: 56860532c6171255869f02a0960f55d155184a46

Author: janicduplessis

Libraries/Blob/RCTBlobManager.mm

@@ -258,6 +258,9 @@ - (BOOL)canHandleNetworkingResponse:(NSString *)responseType
 
 - (id)handleNetworkingResponse:(NSURLResponse *)response data:(NSData *)data
 {
+  // An empty body will have nil for data, in this case we need to return
+  // an empty blob as per the XMLHttpRequest spec.
+  data = data ?: [NSData new];
   return @{
     @"blobId": [self store:data],
     @"offset": @0,

Libraries/Network/RCTNetworking.mm

@@ -439,10 +439,6 @@ - (void)sendData:(NSData *)data
 {
   RCTAssertThread(_methodQueue, @"sendData: must be called on method queue");
 
-  if (data.length == 0) {
-    return;
-  }
-
   id responseData = nil;
   for (id<RCTNetworkingResponseHandler> handler in _responseHandlers) {
     if ([handler canHandleNetworkingResponse:responseType]) {
@@ -452,6 +448,10 @@ - (void)sendData:(NSData *)data
   }
 
   if (!responseData) {
+    if (data.length == 0) {
+      return;
+    }
+
     if ([responseType isEqualToString:@"text"]) {
       // No carry storage is required here because the entire data has been loaded.
       responseData = [RCTNetworking decodeTextData:data fromResponse:task.response withCarryData:nil];