Fix MUI error when selecting a date as for StartDate filter

[ferishili]

This PR fixes #636,

Upon selecting the StartDate filter and adding selecting a date, an error occurs in MUI level, which can be seen in the devtool console!

It is solved simply by making the endDate and startDate value so called "uncontrolled?" by making it optional as to force passing null!

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/652/2024-06-05_14-30-20/ .
It might take a few minutes for it to become available.

[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-652

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-652

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.

[lkiesow]

Probably makes sense to merge #654 first

[Arnei]

Odd fix, but can't argue with results!