OOUIHTMLForm: Don't output an empty hidden LabelWidget if not needed

OOUIHTMLForm: Don't output an empty hidden LabelWidget if not needed

The hidden element is only needed if either a header is set or the form
has errors. If both does not apply, the element isn't needed and should
be omitted instead of hidden with `display:none`.

It's also not needed to add a LabelWidget for the header, if the form has
errors, only, but no header (which would result in an empty space between
the header and the fieldset legend).

Follow up: Ifbf38878d41906184f97169b22002f788711a311

Change-Id: I4ffefdc50d435590d25a7dfc202228920dd801ae