Fix add-event dialog console warnings

[owi92]

Fixes a "missing key prop" and an "invalid prop type" error by adding that key and changing the prop type.
Not sure what to do about the missing i18next keys though.

Closes #550

[github-actions]

Use docker or podman to test this pull request locally.

Run test server using develop.opencast.org as backend:

podman run --rm -it -p 127.0.0.1:3000:3000 ghcr.io/opencast/opencast-admin-interface:pr-583

Specify a different backend like stable.opencast.org:

podman run --rm -it -p 127.0.0.1:3000:3000 -e PROXY_TARGET=https://stable.opencast.org ghcr.io/opencast/opencast-admin-interface:pr-583

It may take a few seconds for the interface to spin up.
It will then be available at http://127.0.0.1:3000.
For more options you can pass on to the proxy, take a look at the README.md.

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/583/2024-06-04_17-01-32/ .
It might take a few minutes for it to become available.

[ziegenberg]

Not sure what to do about the missing i18next keys though.

That's a task for another issue/pull request because the i18next::translator: missingKey en-US translation <empty string> <empty string> 2 i18next.js:13:48 messages are all over the place and not limited to the add-event dialogue.