Added check to see if the variable file is empty

The `pw-token-gen.py` script will fail if used against an empty file.
This change adds a None type check and returns an exit error and
messaage if the loaded value is None.

Closes-Bug: 1446338
Change-Id: I83cb9d1ef312f870f555c6de5f42815ba40e895e

scripts/pw-token-gen.py

@@ -157,6 +157,12 @@ def main():
     with open(user_vars_file, 'rb') as f:
         user_vars = yaml.safe_load(f.read())
 
+    if not user_vars:
+        raise SystemExit(
+            'FAIL: The variable file provided [ %s ] is empty.'
+            % user_vars_file
+        )
+
     changed = False
     generator = CredentialGenerator()
     for entry, value in user_vars.iteritems():