Add function to ensure a non-empty string.

docs/CHANGELOG.md

@@ -21,4 +21,5 @@
 - Add class for Python modules
 - Add feature to get the signature representation of a Python function argument
 - Add class for Python docstrings
-- Add feature to parse Python docstrings
+- Add feature to parse Python docstrings
+- Add function to ensure a non-empty string.