fix: deleting gist files

[codebytere]

Closes #1457.

The previous logic appears to have been based on a misconception:

Gist files are deleted by setting content to an empty string.

Per docs:

To delete a file, set the whole file to null. For example: hello.py : null. The file will also be deleted if the specified object does not contain at least one of content or filename.

[coveralls]

coverage: 88.325% (+1.2%) from 87.085% when pulling de22364 on fix-deleting-gist-files into d7c49b6 on main.