Call completion block if writerObjectContext doesn't have changes to save

Hi,

In the `- (void)saveContextAndWait:(BOOL)wait completion:(void (^)(NSError *error))completion` method of `MDMPersistenceController` it's possible for the `managedObjectContext` to have changes while the `writerObjectContext` does not; in that case the completion block is never called. This PR ensures the completion block is called in that scenario.

Cheers!