Fix setting disclosure input to null in kirby-item

[bdyhsj]

Which issue does this PR close?

This PR closes #2905

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Are there any additional context?

It has been tested to work by modifying the showcase example.
This modification has not been added to the PR as it is a very minor change.

Checklist:

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Reminders

• Make sure you have implemented tests following the guidelines in: "The good: Test".
• Make sure you have updated the cookbook with examples and showcases (for bug fixes, enhancements & new components).

Review

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When the pull request has been approved it will be merged to develop by Team Kirby.

[RasmusKjeldgaard]

Remember to add it to the API table too!

[bdyhsj]

Should i include a test to verify that the disclosure is removed correctly after setting to null?

[RasmusKjeldgaard]

Should i include a test to verify that the disclosure is removed correctly after setting to null?

Good idea, would be a nice addition to ensure that e.g. the fallback icon for kirby-icon is not shown if null is supplied.

[RasmusKjeldgaard]

🎉