PR: Adjust calls to use has_filenames return value of None

[csabella]

Fixes #5776.

The code calling has_filename and is_file_opened was sometimes checking for a None or False value using if not index. This allowed the code to skip index 0 as a valid value.

I also added docstrings because I think it's confusing that a function called has_filename would return an integer and not a boolean. Additionally, is_file_opened can return a boolean or an integer depending on the parameter value.

[ccordoba12]

Looks good to me, thanks @csabella!