frontend: don't assume all additional repos are copr://

The get_copr_by_repo_safe() is designed to return None.  It calls
helpers.copr_repo_fullname() which may return None as well, so when it
happens we need to return None (not try to None.split()).

Fixes: #1142

frontend/coprs_frontend/coprs/logic/complex_logic.py

@@ -151,7 +151,10 @@ def get_copr_by_owner_safe(owner_name, copr_name, **kwargs):
 
     @staticmethod
     def get_copr_by_repo_safe(repo_url):
-        owner, copr = helpers.copr_repo_fullname(repo_url).split("/")
+        copr_repo = helpers.copr_repo_fullname(repo_url)
+        if not copr_repo:
+            return None
+        owner, copr = copr_repo.split("/")
         return ComplexLogic.get_copr_by_owner_safe(owner, copr)
 
     @staticmethod