fixed blobmanager inserting nil when there's no mime type

Summary:
Upon handling symbolication response when a redbox occurs, the blob manager inserted `nil` mime type because the response header doesn't specify any. This crashed the app with exception:

```
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[4]'
terminating with uncaught exception of type NSException
```

Reviewed By: shergin

Differential Revision: D6945632

fbshipit-source-id: 9298bd2674447106763c73e1a5035417bd30f29c

Libraries/Blob/RCTBlobManager.mm

@@ -13,6 +13,7 @@
 
 #import <React/RCTConvert.h>
 #import <React/RCTNetworking.h>
+#import <React/RCTUtils.h>
 #import <React/RCTWebSocketModule.h>
 
 static NSString *const kBlobURIScheme = @"blob";
@@ -263,8 +264,8 @@ - (id)handleNetworkingResponse:(NSURLResponse *)response data:(NSData *)data
     @"blobId": [self store:data],
     @"offset": @0,
     @"size": @(data.length),
-    @"name": [response suggestedFilename],
-    @"type": [response MIMEType],
+    @"name": RCTNullIfNil([response suggestedFilename]),
+    @"type": RCTNullIfNil([response MIMEType]),
   };
 }