i18n: Fix backtrace when missing input kwarg

[xclaesse]

When input kwarg is missing in i18n.merge_file() it was crashing with a
backtrace because of kwargs['input'][0]. That code was useless anyway
because CustomTarget now uses first output as default name which is what
we need here.

[xclaesse]

With this PR, test cases/frameworks/6 gettext prints for example: Generating data/test.desktop with a custom command.

[dcbaker]

That's going to to change the name of the target though, right?

[eli-schwartz]

It still generates the same filename either way... The only part of the ninja rule which is affected is the description field.

[codecov]

Codecov Report

Merging #9457 (df23cb9) into master (abaa980) will decrease coverage by 0.01%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master    #9457      +/-   ##
==========================================
- Coverage   67.31%   67.30%   -0.02%     
==========================================
  Files         396      396              
  Lines       85436    85454      +18     
  Branches    17655    17657       +2     
==========================================
- Hits        57514    57513       -1     
- Misses      23237    23252      +15     
- Partials     4685     4689       +4     

Impacted Files	Coverage Δ
mesonbuild/compilers/detect.py	56.35% <25.00%> (-0.24%)	⬇️
mesonbuild/envconfig.py	74.16% <62.50%> (-0.97%)	⬇️
mesonbuild/modules/i18n.py	76.23% <100.00%> (-2.15%)	⬇️
mesonbuild/scripts/vcstagger.py	87.50% <0.00%> (-4.17%)	⬇️
modules/i18n.py	76.23% <0.00%> (-2.15%)	⬇️
envconfig.py	73.20% <0.00%> (-0.89%)	⬇️
compilers/detect.py	41.85% <0.00%> (-0.18%)	⬇️
modules/python.py	63.22% <0.00%> (ø)
mesonbuild/modules/python.py	65.74% <0.00%> (ø)

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[xclaesse]

It change the target name for VS backend, and for "meson compile", but not for ninja which uses the output filename. But we never gave a guarantee how we name targets, especially not stuff like this, IMHO.

[xclaesse]

@dcbaker @eli-schwartz I think this should be fine to merge. Maybe not in stable branch then?