Skip empty files during mix format

[ak3nji]

Previously it was adding an empty line to empty files during the mix format task.
This PR makes it so empty files are skipped and remain unchanged.

Fixes #11800

[josevalim]

Thank you for the PR! I believe we should make this change up in the Code.format_string code and similar, and make it sure that formatting "" always returns "".

[ak3nji]

Thank you for the PR! I believe we should make this change up in the Code.format_string code and similar, and make it sure that formatting "" always returns "".

I took a look at Code.format_string! and it was already formatting "" to "". The bug was in Format.elixir_format/2 which was always adding a trailing \n for every string, including the empty string from an empty file.

[josevalim]

Do we still need this change, given the change to elixir_format?

[ak3nji]

This should skip empty files when formatting, but the change to elixir_format skips formatting empty lines, and an empty file is just an empty line. Removed this piece of code from the PR as it was redundant. Thanks!

[josevalim]

The bug was in Format.elixir_format/2 which was always adding a trailing \n for every string, including the empty string from an empty file.

I see, makes sense!

[josevalim]

💚 💙 💜 💛 ❤️

[eksperimental]

Thank you @ak3nji for looking into it.
After the fix, I 'm playing around with it and found another edge case.
If you create a file only with a space it will format it as \n.
I'm wondering if we should trim the content of the file and then format it. In this case if we do that, it will return an empty string.
I think both results are OK, just mentioning it to see if there is a rationale that is more suitable than the other.