Improved null url handling

Summary:
public
Attempting to load an undefined URL via XMLHttpRequest produced a confusing error deep within the network layer. This diff improves the networking stack to catch such errors earlier, and also adds a helpful error in the JS layer.

Fixes https://github.com/facebook/react-native/issues/4558

Reviewed By: javache

Differential Revision: D2811080

fb-gh-sync-id: 1837427e1080a0308f2c4f9a8a42bce2e041fb48

Libraries/Network/RCTNetworking.m

@@ -136,6 +136,10 @@ RCT_EXPORT_MODULE()
 
 - (id<RCTURLRequestHandler>)handlerForRequest:(NSURLRequest *)request
 {
+  if (!request.URL) {
+    return nil;
+  }
+
   if (!_handlers) {
 
     // get handlers

Libraries/Network/XMLHttpRequestBase.js

@@ -179,6 +179,9 @@ class XMLHttpRequestBase {
       // async is default
       throw new Error('Synchronous http requests are not supported');
     }
+    if (!url) {
+      throw new Error('Cannot load an empty url');
+    }
     this._reset();
     this._method = method;
     this._url = url;