fix(mimetype): Fix returning value when finding existing mimetype in MimeType Loader

[tcitworld]

Fixes https://github.com/nextcloud/server/pull/35744/files#r1184644610

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)