Allow setting anonnotice to "" to prevent fallback to sitenotice

Allow setting anonnotice to "" to prevent fallback to sitenotice

Previously we only used Message::isDisabled (non-existent, empty string,
or "-") to decide whether to show a notice. If 'anonnotice' was disabled
'sitenotice' was shown instead.

When addressing logged-in users only, wikis typically use 'sitenotice'
and then put invisible content in 'anonnotice' (so that it shadows the
'sitenotice'). Now that the DismissableSiteNotice extension supports
closing of the notice for anonymous users (as of I87df3301c), this
becomes problematic as it has no way of knowing the notice was meant to
be invisible (and thus renders bogus "[close]" to all users).

This supersedes hacks such as <p></p> or <span></span>. Instead, the
empty string can now be used to have no anonnotice content, but also
don't show the 'sitenotice'.

Disabling with '-' (default) can still used to fallback to 'sitenotice'.

Also changed the code to, for consistency, allow hiding $wgSiteNotice
for all users by setting the sitenotice message to "-".

Bug: T94536
Change-Id: I11b8b883d480d0e07d8b395dd92360cb15de7c5b