Fix "promise/plugin: Implement check_file_state promise plugin"

If there is no url parameter the promise fails, so simply switch to empty.

slapos/promise/plugin/check_file_state.py

@@ -19,7 +19,7 @@ class RunPromise(GenericPromise):
 
     filename = self.getConfig('filename')
     state = self.getConfig('state')
-    url = self.getConfig('url').strip()
+    url = (self.getConfig('url') or '').strip()
 
     try:
       with open(filename) as f: