Save pre/post-scripts paths with None instead of "" on reset

[EmoonX]

Description

So far, whenever resetting a pre/post-script path for a program, the value "" (empty string) was used. In such cases, the checks below have been useless and the program would always try to append/prepend sh '' (which thankfully didn't do anything):

if post_script is not None:
    command = f"{command} ; sh '{post_script}'"

if pre_script is not None:
    command = f"sh '{pre_script}' ; {command}"

Now, and for consistency, settings are always saved with the value None instead to indicate their absence.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update