Fix condition if...else in getDB() & PHPDoc comment for getUserDB()

Fix condition if...else in getDB() &  PHPDoc comment for getUserDB()

So the conditional check should by default return $this->mDb if it's not
null, so, the else seems not to be needed(?). If we have a database handle
to process the current batch, $this->getDB() will return IMaintainableDatabase
but if it's not available (null), a call to $this->getDB() will return an
instance of \Wikimedia\Rdbms\Database is returned instead.

In accordance with the documentation (phpdoc), update the method getUserDB()
to be compliant with callers return type.

Change-Id: I95f3407dd2ffe8e4a1ad7a70be86b6cf3b65ff50