fix: handle null mime type in preview generation

[boris324]

Fixes #58784

When generating previews for images with unusual dimensions (e.g. 18x4312 GIF), IImage::dataMimeType() can return null. This null value gets passed to Preview::setMimeType(string), causing a TypeError that crashes the entire occ preview:generate-all batch.

This adds null checks for the MIME type returned by dataMimeType() in both preview generation paths:

• In generateProviderPreview(): skip to the next provider with a warning log
• In generatePreview(): throw NotFoundException so the file is skipped gracefully

Both are already handled by callers, so one bad file no longer terminates the batch.

[kesselb]

Still needed after #58793?

[solracsf]

Why 2 PR? Is this some AI bot? Just asking 😝
#58802

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[CarlSchwan]

/backport to stable33

[CarlSchwan]

/backport to stable34

[artonge]

Blocking as https://github.com/nextcloud/server/pull/58793/changes seems to address the issue for generatePreview