WikitextContent methods replaceSection() (used when adding a section
to an existing page) and addSectionHeader() (used when creating a new
page) behaved inconsistently - the former would omit the heading
syntax when the section title was empty, but the latter would not.
They both omit it now.
Some callers of addSectionHeader() handled this case, but others did
not, which caused T311489. (I am removing the checks now, since the
change makes them redundant.)
Bug: T311489
Change-Id: Icd59bcf2b75bf50865f19fac92bddabe0c183dcc
add6c49e01