Infer extension name from typeOf if data-mw not present

For bug T75121. If there's no data-mw or data-parsoid.src from
which to recover information about the extension, try at least
getting its name from the typeOf attribute. This will produce an
empty extension tag, but it should be better than an error.

Change-Id: I3a65c28e570aafd99ed17598b1c1b709fd1a8003