lib: Texinfo: Translate partial menu node names.

[Apteryks]

Fixes https://issues.guix.gnu.org/64881.

• lib/Locale/Po4a/Texinfo.pm (translate_buffer_menuentry): Refine regexp, so that it matches menu entries lacking a description. Only call 'translate_buffer' on the description if it was provided.
• t/fmt/texinfo/partialmenus.trans: New file.
• t/fmt/texinfo/partialmenus.texi: Likewise.
• t/fmt/texinfo/partialmenus.pot: Likewise.
• t/fmt/texinfo/partialmenus.po: Likewise.
• t/fmt/texinfo/partialmenus.norm: Likewise.
• t/fmt-texinfo.t: Register the new 'partialmenus' test.

[mquinson]

Thanks a lot, merged! Your help is particularly precious because I'm far from being able to update/fix the Texinfo module...

Just a small comment: please also update the NEWS file to document your change, and my pleasure will be perfect.

Thanks again, Mt

[Apteryks]

Noted for future contributions, thanks for the review/merge!