Disable writing command line history if entry is empty

[weirddan455]

Follow up to #2629 and #2630

Command history should be disabled if the config entry is empty. This is a regression caused by my previous PR. Added missing empty check.

Likely this just caused an erroneous warning message as it tried to create a file with the same name as the user's config directory, which will fail.

[johnnovak]

@weirddan455 I have given you write access to the dosbox-staging repo; now you should be able to create your own branches with the wd/ prefix without having to fork. You should also be able to force push into them and delete your branches. Makes life so much easier! 🎉

Please pay attention to the wd/ prefix — that must be present in branch names created by you.

Congratulations! 🎉 🍻

Merging this now, then you can try out your new powers in your next PR 😎