Expected results come from 11.0.x branch.

Also remove `ExpressionNode.Reset` as it was doing unneeded stuff and should just be the same as `SetSource(AvaloniaProperty.UnsetValue, null);`.

The previous design assumed that a source of `null` was an invalid input to all types of expression nodes. It turned out that there was a single exception to this rule: the `!` operator can in fact operate on a null value. With this new design we instead have to explicitly check for a null value in every override of `OnSourceChanged ` except in `LogicalNotNode`.

Due to this change, we also have to distinguish between `null` and `(unset)` in `ExpressionNode.SetSource` as well.

Fixes #16071