Add structure test for ContentHandler::makeEmptyContent()

Add structure test for ContentHandler::makeEmptyContent()

This adds a structure test that verifies all registered ContentHandlers
will return a valid content object (as determined by
Content::isValid()), when makeEmptyContent() is called. Additionally, if
the handler extends TextContentHandler, it will verify it is an instance
of TextContentHandler::getContentClass().

This is a structure test so it is run for content handlers created by
extensions as well.

Change-Id: Idd67d824f2f7cc1819b1d8b186be058e63c4513d