Initialize Request.creator with the empty string instead of None

Old obs versions have no creator attribute. In this case, string
formatting operations will fail if the "creator" attribute is
initialized with None (affects some codepaths).
An alternative would be to officially resurrect the deprecated
"Request.get_creator" method, which could return something like
"unknown" if no creator information is present.

This is a follow-up fix for the commits d68507f and 7d54b5c.

osc/core.py

@@ -2684,7 +2684,7 @@ class Request:
     def _init_attributes(self):
         """initialize attributes with default values"""
         self.reqid = None
-        self.creator = None
+        self.creator = ''
         self.title = ''
         self.description = ''
         self.priority = None