Improve display of empty files

[silverwind]

1. Empty files were not rendering anything in their body, add a "Empty File" text to them, both markup and plaintext.
2. Since go 1.19 it's possible to compare against nil in templates, so remove the NumLinesSet workaround.
3. Always show file size, even at 0 B.
4. Always show file size and line count for markdown files, even when rendered.

Diff without whitespace

[silverwind]

This has a few good fixes but I'm not sure I'm interested in working on this further currently, especially to go parts.

Maybe @wxiaoguang you want to pick it up where it left off, the test files are all in my test repo:

• https://try.gitea.io/silverwind/symlink-test/src/branch/master/empty.md
• https://try.gitea.io/silverwind/symlink-test/src/branch/master/empty.txt
• https://try.gitea.io/silverwind/symlink-test/src/branch/master/playground/README.md

[wxiaoguang]

I will try to rework it if I get time

[wxiaoguang]

-> Improve detecting empty files #31332

[silverwind]

Replaced by #31332