fix: add None support for ClientUser edit

[ItsRqtl]

Summary

Fixed AttributeError: 'NoneType' object has no attribute 'startswith' in _bytes_to_base64_data when using None to reset avatar.

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.
• I have updated the changelog to include these changes.

[codecov]

Codecov Report

Merging #1994 (c4e6b75) into master (982c5d8) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1994      +/-   ##
==========================================
- Coverage   33.33%   33.33%   -0.01%     
==========================================
  Files          97       97              
  Lines       18898    18900       +2     
==========================================
  Hits         6300     6300              
- Misses      12598    12600       +2     

Flag	Coverage Δ
macos-latest-3.10	33.31% <0.00%> (-0.01%)	⬇️
macos-latest-3.11	33.31% <0.00%> (-0.01%)	⬇️
macos-latest-3.8	33.32% <0.00%> (-0.01%)	⬇️
macos-latest-3.9	33.32% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.10	33.31% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.11	33.31% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.8	33.32% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.9	33.32% <0.00%> (-0.01%)	⬇️
windows-latest-3.10	33.31% <0.00%> (-0.01%)	⬇️
windows-latest-3.11	33.31% <0.00%> (-0.01%)	⬇️
windows-latest-3.8	33.32% <0.00%> (-0.01%)	⬇️
windows-latest-3.9	33.32% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
discord/user.py	40.65% <0.00%> (-0.46%)	⬇️

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[plun1331]

Please add a changelog.

[Lulalaby]

conflicts

M