Header should render even if no header exists yet

Catchable fatal error: Argument 1 passed to
Flow\Templating::renderHeader() must be an instance of
Flow\Model\Header, null given

Template properly handles no header (it shows a "This talk
page currently has no header." message) so there is no
reason to not allow null-for-header in there.

Bug: 62159
Change-Id: Idc92fed33de8c7be7230023a10a95758635809ca
Merge "Header should render even if no header exists yet"