[main] Fix: Keep 'done' status when updating archived card

[theoholl]

• Resolves: The card completion mark is reset if changes were made to the completed card #7294
• Target version: main

Summary

The update method of the CardController now handles the optional (and nullable) done parameter. When a card is marked as done, and the card updated, the 'done' status is not cleared anymore.

Screencast.Fix.mp4

[grnd-alt]

this change is correct in my view and approved as a fix.
But the behavior of cardService is very weird here.
The OptionalNullableValue doc states that done == null means the value has not been set and should not be updated, and done->getValue() == null means the value has been set and should be updated to be null.

Now the actual CardService behavior is to always just set the value though (as seen here: https://github.com/nextcloud/deck/blob/main/lib/Service/CardService.php#L297).
Which means when done is not included in the request it will be set to null. Which currently is not a problem, as we always seem to send it but is still very unexpected behavior in my eyes.
This change was done for the android app a while ago in this PR: #5491
But I don't really know why.
Maybe @stefan-niedermann could elaborate why this was required exactly, and if we can make done behave like all the other optional params to that function (ignore when null)