feat: do not load config file when path is empty

[litetex]

Fixes #794

[sysadmind]

This input is almost guaranteed to never be empty. It has a default in the flag. What are you trying to achieve with this change because I don't think this achieves it?

[litetex]

This input is almost guaranteed to never be empty. It has a default in the flag. What are you trying to achieve with this change because I don't think this achieves it?

Hi, thank you for taking the time and asking.

I'm basically running the quickstart setup without any config file.

However this results in the warning: Error opening config file... because it does not exist in my setup.

I want to get rid of the warning because - you know - it's a warning and warning's require attention/a fix in some way.

I thought the simplest way was by setting --config.file=''. However the warning then still happens because there is no empty string check (therefore this PR) or any other way to disable using a config file.

Hope that this helps :)

[mator]

could use filepath.Clean(f) instead of a plain "f"