TimeZone::from_posix_tz: Treat empty TZ variable as UTC

[drinkcat]

This is not technically POSIX, but glibc and musl behave that way: e.g. TZ= date returns a UTC date.

Fixes #1690.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.67%. Comparing base (dc068f0) to head (ce183c2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1691      +/-   ##
==========================================
+ Coverage   90.62%   90.67%   +0.04%     
==========================================
  Files          38       38              
  Lines       16871    16872       +1     
==========================================
+ Hits        15290    15299       +9     
+ Misses       1581     1573       -8     

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[djc]

Thanks!

[drinkcat]

@djc not terribly urgent, but any chance we could get a release with this sometimes soon? Would be useful for uutils/coreutils. Thanks!

[drinkcat]

@djc still not terribly urgent, but we'd really like to get this and #1693 for uutils/coreutils. Any chance we can get a new release? ,-) Thanks!!

[djc]

@djc still not terribly urgent, but we'd really like to get this and #1693 for uutils/coreutils. Any chance we can get a new release? ,-) Thanks!!

See #1698.