Remove unnecessary packager running check when saved JSLocation is empty

Summary:
Changelog:
When the JSLocation is nil, checking whether the address is running is unnesarry and wasting time, adding a JSLocation length check to mitigate that.

Reviewed By: cpojer

Differential Revision: D22644574

fbshipit-source-id: c51fc1a8976ebc25cba2653581e1bfa479a1d70d

React/Base/RCTBundleURLProvider.mm

@@ -126,7 +126,7 @@ - (NSString *)packagerServerHost
 {
   NSString *location = [self jsLocation];
 #if RCT_DEV_MENU
-  if (![RCTBundleURLProvider isPackagerRunning:location]) {
+  if ([location length] && ![RCTBundleURLProvider isPackagerRunning:location]) {
     location = nil;
   }
 #endif