Skip empty message when writing mo file

[cedk]

Instead of using the msgid, this allows to have gettext fallback to
retrieve the translation. Otherwise gettext will anyway return msgid.

[codecov-io]

Codecov Report

Merging #564 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #564 +/- ##
==========================================
+ Coverage 90.23% 90.31% +0.07% 
==========================================
Files 24 24 Lines 4045 4128 +83 ==========================================
+ Hits 3650 3728 +78 - Misses 395 400 +5

Impacted Files	Coverage Δ
babel/messages/mofile.py	90.1% <100%> (-0.32%)	⬇️
babel/_compat.py	98.03% <0%> (-1.97%)	⬇️
babel/messages/pofile.py	96.19% <0%> (-0.66%)	⬇️
babel/numbers.py	97.67% <0%> (-0.34%)	⬇️
babel/localtime/_unix.py	23.28% <0%> (ø)	⬆️
babel/messages/catalog.py	94.69% <0%> (+0.06%)	⬆️
babel/messages/frontend.py	86.34% <0%> (+0.08%)	⬆️
babel/util.py	92.4% <0%> (+0.29%)	⬆️
babel/plural.py	96.04% <0%> (+0.35%)	⬆️

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[akx]

Is this the behavior of the GNU gettext .mo utilities too?

I'd like to see a test added, anyway. :)

[cedk]

I added a test.
Here is the place where msgfmt from GNU Gettext does the same: https://github.com/autotools-mirror/gettext/blob/master/gettext-tools/src/msgfmt.c#L1296

[akx]

Thank you for the test and the verification! (And thanks for the code too, of course.)