compress: treat an empty BORG_ZSTD_MT_WORKERS as unset

[ThomasWaldmann]

An empty value made borg fail:

$ BORG_ZSTD_MT_WORKERS= borg benchmark cpu --compressing --data ~/silesia
Compression ==========================================================
zstd,-4 (2MiB)             failed: Error: Error: BORG_ZSTD_MT_WORKERS must be an integer, but is: ''

An env var set to an empty string usually means "not configured" - e.g. when a wrapper
script or a systemd unit passes a variable through unconditionally. So use the default
values in that case instead of erroring out (whitespace-only values are treated the same
way). Non-empty invalid values (yes, 4x, 1.5, -1) still raise as before.

Also updated the test and the BORG_ZSTD_MT_WORKERS docs (borg help env_vars and
docs/usage/general/environment.rst.inc).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.70%. Comparing base (94bd5bd) to head (3e7da72).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10323      +/-   ##
==========================================
+ Coverage   87.65%   87.70%   +0.04%     
==========================================
  Files         103      103              
  Lines       18716    18716              
  Branches     2881     2881              
==========================================
+ Hits        16405    16414       +9     
+ Misses       1609     1600       -9     
  Partials      702      702              

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