refactor(frontend): Use nullish coalescing operator

[woxtu]

What

||を??に代えてESLintの@typescript-eslint/prefer-nullish-coalescingルール違反を解決します。

/***/misskey/packages/frontend/src/components/global/MkMisskeyFlavoredMarkdown.ts
  259:123  error    Prefer using nullish coalescing operator (`??`) instead of a logical or (`||`), as it is a safer operator  @typescript-eslint/prefer-nullish-coalescing

Why

エラーが消えると嬉しい。

Additional info (optional)

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests

[codecov]

Codecov Report

Merging #11294 (4018ca6) into develop (5453710) will decrease coverage by 0.13%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop   #11294      +/-   ##
===========================================
- Coverage    77.82%   77.70%   -0.13%     
===========================================
  Files          916      916              
  Lines        92525    92525              
  Branches      7768     7739      -29     
===========================================
- Hits         72008    71895     -113     
- Misses       20517    20630     +113     

Impacted Files	Coverage Δ
...src/components/global/MkMisskeyFlavoredMarkdown.ts	29.15% <0.00%> (ø)

... and 17 files with indirect coverage changes

[syuilo]

🙏🏻