Don't crash on null edit

Null edits won't include 'newtimestamp' in the response

mwclient/page.py

@@ -202,7 +202,9 @@ class Page(object):
                 self.handle_edit_error(e, summary)
 
         if result['edit'].get('result') == 'Success':
-            self.last_rev_time = client.parse_timestamp(result['edit'].get('newtimestamp'))
+            lrt = result['edit'].get('newtimestamp')
+            if lrt:  # It's not included if no change was made
+                self.last_rev_time = client.parse_timestamp(lrt)
         return result['edit']
 
     def handle_edit_error(self, e, summary):