Allow a TitleInputWidget user to decide, if an empty value should be validated

Allow a TitleInputWidget user to decide, if an empty value should be validated

For some use cases an empty value is valid, too, or at least, it's not a reason to
mark the form input red. Special:Search, e.g.. This change implements a new config
for MediaWiki\Widgets\TitleInputWidget, validate, which allows a user of this
widget to decide, if the value of the input type should be validated (empty -> flagged
red).

Extra points:
 * Fix php notice errors for previously added configuration
 * Added doc for previously added configuration

Bug: T106946
Change-Id: I732a2f56a2375d8c708e3b295996187ee209f1a6