Allow parsing .po files that have an extant but empty Language header

[akx]

Fixes #1087.

Some tools apparently generate somewhat strange Language MIME headers, so we'll treat them like the Language was unset.

Trying catalog.locale = "" by hand will still happily crash; this only affects MIME parsing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.09%. Comparing base (2f87363) to head (3abacbc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1101   +/-   ##
=======================================
  Coverage   91.09%   91.09%           
=======================================
  Files          26       26           
  Lines        4470     4470           
=======================================
  Hits         4072     4072           
  Misses        398      398           

Flag	Coverage Δ
macos-12-3.10	89.91% <100.00%> (ø)
macos-12-3.11	89.91% <100.00%> (ø)
macos-12-3.12	90.20% <100.00%> (ø)
macos-12-3.8	89.83% <100.00%> (ø)
macos-12-3.9	89.90% <100.00%> (ø)
macos-12-pypy3.10	89.91% <100.00%> (ø)
ubuntu-22.04-3.10	89.93% <100.00%> (ø)
ubuntu-22.04-3.11	89.93% <100.00%> (ø)
ubuntu-22.04-3.12	90.22% <100.00%> (ø)
ubuntu-22.04-3.8	89.85% <100.00%> (ø)
ubuntu-22.04-3.9	89.92% <100.00%> (ø)
ubuntu-22.04-pypy3.10	89.93% <100.00%> (ø)
windows-2022-3.10	90.27% <100.00%> (ø)
windows-2022-3.11	90.27% <100.00%> (ø)
windows-2022-3.12	90.57% <100.00%> (ø)
windows-2022-3.8	90.20% <100.00%> (ø)
windows-2022-3.9	90.27% <100.00%> (ø)
windows-2022-pypy3.10	90.27% <100.00%> (ø)

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[tomasr8]

LGTM!