[@mantine/core] Allow value reset in SegmentedControl

[gl3nn]

Currently it's not possible to deselect/reset the value of the SegmentedControl. You can start out with an empty value, if you take for instance the empty string as value and make sure that in the data property it doesn't map to anything. If however you then select a value, and then change the value state back to the empty string, the SegmentedControl will still visually show your last selected value to be selected (however data-selected=true will no longer be present).

My PR here fixes this behavior and allows you, with a controlled value, to make the component unselected / without anything selected.