HTMLFormField: Treat weird ' ' labels as empty in OOUI mode

HTMLFormField: Treat weird ' ' labels as empty in OOUI mode

I have no idea where this convention came from, but we had them in
core until 265ff105aaa814c828a35e599ed35d0b0bb00e53 and they still
appear widely in extensions. The non-OOUI code also has special
handling for it (a label equalling ' ' is treated as raw HTML
even when not marked as such). In OOUI-style "vertical" forms these
fake labels cause a lot of unnecessary white space to appear, so let's
just not display them.

Change-Id: I45559fa69dc1ae4b9d048445e27a24815fe93b6d