remap-rescan: prevent empty/damaged repos to break the remap operation.

rhodecode/model/db.py

@@ -2276,7 +2276,7 b' class Repository(Base, BaseModel):'
             # use no-cache version here
             scm_repo = self.scm_instance(cache=False, config=config)
 
-            empty = scm_repo.is_empty()
+            empty = not scm_repo or scm_repo.is_empty()
             if not empty:
                 cs_cache = scm_repo.get_commit(
                     pre_load=["author", "date", "message", "parents"])