[MannXo]

With TMPDIR set to the empty string, mktemp -t T, mktemp -d -t T, mktemp --tmpdir T and mktemp -p DIR -t T created the file in the current directory and printed a bare name. GNU treats an empty TMPDIR as unset.

The template arm of Options::from now ignores an empty TMPDIR for -t, and its fallback goes through get_tmpdir_env_or_default(), the helper the no-template path already uses. That also lets -p DIR apply when TMPDIR is empty, and removes the duplicated WASI branch.

$ TMPDIR= mktemp -t fooXXX
/tmp/foo3E8
$ TMPDIR= mktemp -p /tmp/cu-other -t fooXXX
/tmp/cu-other/foorbO

Before, these printed foorx8 and fooUlY, created in the current directory.

test_empty_tmpdir_env_var_with_template covers all four forms and fails on main.

Closes #15790