Adds an empty test case to the test case controller and nullifies the te...

...st cases controller data source on category delete.

What it does
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Fixes a bug where test cases and test case details would persist after deleting their parent category.

How to test
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1. Create a category
2. Add some test cases
3. Delete the category
4. Test cases should empty and the header for the test case details will revert to "No Test Case Selected"

- [x] Review
- [x] Test