UI-Component: fix null value input-handling for group in group

[iszmais]

https://mantis.ilias.de/view.php?id=48017

[thibsy]

@iszmais I deliberately shifted the null check outside of isClientSideValueOk(). I know that for groups this is somewhat special, since are not sharing the same symbol, but could you find a solution that leaves this function (or the equivalent with _*) as is? Otherwise this would (to my knowledge) be the only case where null is checked inside this method. Kind regards, @thibsy (as UI coordinator)

[iszmais]

@thibsy Thank you for your feedback.

After your feedback I tested this for other inputs and came to the conclusion that this error occurs for every input in groups (not just other groups). Therefore, it's correct that there is currently no other value check that considers null, but its seems that this is indeed a bug, means actually should every check include this.

The origin of the problem is that the group does not call the same validation method as the form itself, therefore, the validation inside the group diverges from the validation on the form layer.

That shouldn't be the case.
So either the group also needs to call a combine checkArg for validation (e.g. $this->checkArg("value", null === $value || $this->isClientSideValueOk($value)....
Or every Input should explicitly handle the null value.

For me, the second solution seems better conceptionally since there are also inputs that dont allow null as a value (see https://mantis.ilias.de/view.php?id=44169) and due to that every input should handle that individually to align the behavous inside and outside of a group.

Greetings,
@iszmais