chore(i18n): change localization attributes' defaults

[Middledot]

Summary

EDIT: This PR makes it so that l10n attrs are by default MISSING to be more API consistent.
I'd call this a minor breaking change.

Information

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed).
• This PR is not a code change (e.g. documentation, README, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.

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Codecov Report

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The diff coverage is 6.25%.

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Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1866   +/-   ##
=========================================
  Coverage          ?   33.27%           
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  Files             ?       97           
  Lines             ?    19022           
  Branches          ?        0           
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  Hits              ?     6329           
  Misses            ?    12693           
  Partials          ?        0           

Flag	Coverage Δ
macos-latest-3.10	33.25% <6.25%> (?)
macos-latest-3.11	33.25% <6.25%> (?)
macos-latest-3.8	33.26% <6.25%> (?)
macos-latest-3.9	33.26% <6.25%> (?)
ubuntu-latest-3.10	33.25% <6.25%> (?)
ubuntu-latest-3.11	33.25% <6.25%> (?)
ubuntu-latest-3.8	33.26% <6.25%> (?)
ubuntu-latest-3.9	33.26% <6.25%> (?)
windows-latest-3.10	33.25% <6.25%> (?)
windows-latest-3.11	33.25% <6.25%> (?)
windows-latest-3.8	33.26% <6.25%> (?)
windows-latest-3.9	33.26% <6.25%> (?)

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Impacted Files	Coverage Δ
discord/commands/core.py	17.99% <0.00%> (ø)
discord/commands/options.py	17.12% <16.66%> (ø)

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[BobDotCom]

IMO, to better reflect the API it should actually default to MISSING. We omit the field (hence "missing") when localizations are not set, we don't send None, nor do we send {}.

[Middledot]

Well, the difference is that using an empty dictionary would remove the redundancy of needing set the dictionary before or while adding a translation and using MISSING would be more API consistent. Personally I made this pr because I found it weird that I had to that

[BobDotCom]

Still not convinced that this change is really necessary IMO

[Lulalaby]

Tbh agreeing with bob

[Middledot]

Mmmh, ok, I'll do that

[Middledot]

I'm waiting for #1867 to be merged first before I update the changelog for this one

.

[Lulalaby]

@Middledot could you in future pls work on a branch.. codeconv is shitting itself bcs fork .-.

[pullapprove4]

Please add a changelog entry