[bugfix] Fix 404 for local audio files with no cover image (#4739)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

Closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4734 by accounting for media attachments without thumbnail when doing `Cached()` check.

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first two checkboxes must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have not used so-called 'AI' to create the proposed changes.
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4739
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

internal/gtsmodel/mediaattachment.go

@@ -54,7 +54,17 @@ func (m *MediaAttachment) IsRemote() bool {
 
 // Cached returns whether MediaAttachment is cached locally.
 func (m *MediaAttachment) Cached() bool {
-	return m.File.Cached() && m.Thumbnail.Cached()
+	// Original file must be cached.
+	//
+	// Thumbnail must either exist and be cached,
+	// or not exist (for audio files with no cover).
+	return m.File.Cached() && (m.Thumbnail.Cached() || !m.HasThumbnail())
+}
+
+// HasThumbnail returns whether MediaAttachment has a thumbnail.
+// Will be false in the case of audio files with no cover image.
+func (m *MediaAttachment) HasThumbnail() bool {
+	return m.FileMeta.Small.Height > 0 && m.FileMeta.Small.Width > 0
 }
 
 // Stub will reset all non-essential attachment