[l10n] Improve Dutch (nl-NL) locale

[nphmuller]

• Update placeholders from:
  • Y (Year) to J (Jaar)
  • hh (Hour) to uu (Uur)
• Fix compound words: Words like start date are a single word in dutch (startdatum).
• The translation key empty indicates an empty state, not the verb "to empty". It's used for aria-valuetext to indicate an empty value.

[mui-bot]

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[nphmuller]

@oliviertassinari would it make sense to cherry pick these changes to the v7 branch? It might take a while before v8 wil be released right?

[oliviertassinari]

@nphmuller We target the release of v8 in March 2025, proof: https://mui.com/blog/mui-x-v8-alpha-zero/. Ok, commit cherry-picked, proof: d72dd79.

Thanks for the fix.