fix: KeyError in EmbedMedia

[OmLanke]

Summary

Fixes KeyError caused due to unprotected dict attribute accessing.

Fixes #2107.

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 #2109 (65eaf0b) into master (96355dd) will not change coverage.
The diff coverage is 40.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2109   +/-   ##
=======================================
  Coverage   33.13%   33.13%           
=======================================
  Files          97       97           
  Lines       19141    19141           
=======================================
  Hits         6342     6342           
  Misses      12799    12799           

Flag	Coverage Δ
macos-latest-3.10	33.11% <40.00%> (ø)
macos-latest-3.11	33.11% <40.00%> (ø)
macos-latest-3.8	33.12% <40.00%> (ø)
macos-latest-3.9	33.12% <40.00%> (ø)
ubuntu-latest-3.10	33.11% <40.00%> (ø)
ubuntu-latest-3.11	33.11% <40.00%> (ø)
ubuntu-latest-3.8	33.12% <40.00%> (ø)
ubuntu-latest-3.9	33.12% <40.00%> (ø)
windows-latest-3.10	33.11% <40.00%> (ø)
windows-latest-3.11	33.11% <40.00%> (ø)
windows-latest-3.8	33.12% <40.00%> (ø)
windows-latest-3.9	33.12% <40.00%> (ø)

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

Impacted Files	Coverage Δ
discord/embeds.py	25.21% <40.00%> (ø)

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[pullapprove4]

Please add a changelog entry