corrected linting errors and a minor bug in cf_to_string

modules/tools.py

@@ -55,9 +55,8 @@ def cf_to_string(cf, key="name", logger=None):
     Converts a dict custom fields to string
     """
     if isinstance(cf, dict):
-        if key:
+        if key in cf:
             return cf[key]
-        else:
         logger.error("Conversion of custom field failed, '%s' not found in cf dict.", key)
         return None
     return cf