Media types: Handle null configured file extensions when populating allowed media types (closes #20620)

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

Fixes: #20620

Description

Seems with legacy data the configuration for fileExtensions under an upload field data type can be null, which we aren't expecting in backend code. So this PR just defensively ensures we are working with an empty collection and never null.

Testing

There's a method using uSync described in the linked issue, but I replicated the problem and verified the fix with the following:

• Modified the stored configuration for the Upload (Audio) data type with:

update umbracoDataType
set config = '{"fileExtensions": null}'
where nodeId = -101

• Restarted to clear caches

• Made a call to the following: /umbraco/management/api/v1/item/media-type/allowed?fileExtension=mp3&skip=0&take=100

  • Before this update the exception reported in the linked issue should be shown
  • After you should get back a collection of data types

• Reset my database with:

update umbracoDataType
set config = '{"fileExtensions":["mp3","weba","oga","opus"]}'
where nodeId = -101

Release

On approval and merge this will go into 16.4. Let me know if you are happy this goes into 17.0 which I would say is OK given we are in the beta period still. Otherwise it'll be 17.0.

[Copilot]

Pull Request Overview

This PR addresses a null reference exception that occurs when legacy data contains null file extensions in the upload field data type configuration. The fix defensively handles null values by ensuring an empty collection is used instead, preventing the application from crashing when querying allowed media types by file extension.

Key Changes:

• Added null-coalescing operator to handle null FileExtensions configuration values in media type editing service

[nikolajlauridsen]

Looks good to me 👍

Given the type of fix this is, I think it's fine to go into 17.0, I'll cherry-pick it 😄

[nikolajlauridsen]

Cherry picked for 17.0 🍒