Enable the creation of empty pages

This change enables the direct creation of empty pages without needing
to use a work-around (such as "{{subst:ns:0}}"). A warning is added as
the message "blankarticle" to request confirmation that the empty page
was meant to be blank. A automatic edit summary has been added when
creating a blank page. The message is: "autosumm-newblank."

The API has been updated to permit the creation of empty pages, when
"text" is null, but not non-existant.

Unit tests have also been added to test these features.

Bug: 57238
Bug: 65206
Change-Id: I3457c36a909d1dbfaeed04a1f0568c69e0ef3386