i18n: filter out empty env variables

[Juul-Mc-Goa]

Make get_locale_from_env POSIX compliant by ignoring empty env variables.

Fixes #11136.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/pr/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[sylvestre]

could you please add a test ? thanks

[Juul-Mc-Goa]

I've added some tests for sort, but it assumes the host has hungarian locale installed.

Otherwise, I guess I could add a tests module inside uucore/src/lib/features/i18n/mod.rs that:

1. uses the unsafe std::env::set_var to set env variables,
2. calls get_locale_from_env and check that it is correct.

I'm not sure if the second solution is any better than the first.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.