fix: set shelf to {} when it's None for whatever reason

Author: itkovian

lib/vsc/utils/cache.py

@@ -169,6 +169,8 @@ def close(self):
                 self.log.error('cannot open the file cache at %s for writing', self.filename)
             else:
                 if self.retain_old:
+                    if self.shelf is None:
+                        self.shelf = {}
                     self.shelf.update(self.new_shelf)
                     self.new_shelf = self.shelf