Don't attempt to pull from empty cache file

Change-Id: Id26e954f347c35703e506abdc8ecf5146f2efb61
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>
Story: 2010155
Task: 45808

reno/loader.py

@@ -73,6 +73,7 @@ class Loader(object):
             with open(self._cache_filename, 'r', encoding=self._encoding) as f:
                 self._cache = yaml.safe_load(f.read())
 
+        if self._cache:
             # Save the cached scanner output to the same attribute
             # it would be in if we had loaded it "live". This
             # simplifies some of the logic in the other methods.