fix media preview_url nullable type

[Kraigo]

1. Description

Some media type may have nullable preview_url property, which didn't reflect in documentation. You change check it with provided media id

https://mstdn.social/api/v1/media/110023777570725915

1.1. Checklist

• [x] The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• [x] I have read the Contributor Guide and followed the process outlined for submitting PRs.
• [x] I have updated/added tests for ALL new/updated/fixed functionality.
• [ ] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [ ] I have updated/added relevant examples in examples.

1.2. Breaking Change

• [ ] Yes, this is a breaking change.
• [x] No, this is not a breaking change.

1.3. Related Issues

[codecov-commenter]

Codecov Report

Merging #150 (2176b72) into main (6d5db71) will not change coverage. The diff coverage is n/a.

:mega: This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@           Coverage Diff           @@
##             main     #150   +/-   ##
=======================================
  Coverage   75.83%   75.83%           
=======================================
  Files         115      115           
  Lines        1254     1254           
=======================================
  Hits          951      951           
  Misses        303      303           

Impacted Files	Coverage Δ
lib/src/service/entities/media_attachment.dart	100.00% <ø> (ø)

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more

[myConsciousness]

Hi @Kraigo ,

Thanks for your contribution!:)