test: Added to z_function test corner cases for empty text and pattern

[jasonjyu]

Description of Change

Added to z_function test corner cases for empty text and pattern

Checklist

• [x] Added description of change
• [ ] Added file name matches File name guidelines
• [x] Added tests and example, test must pass
• [x] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• [x] Relevant documentation/comments is changed or added
• [x] PR title follows semantic commit guidelines
• [x] Search previous suggestions before making a new one, as yours may be a duplicate.
• [x] I acknowledge that all my contributions will be made under the project's license.

Notes: Added to z_function test corner cases for empty text and pattern