Fix two instances of calling function with too few args

Fix two instances of calling function with too few args

At one point SearchIndexFieldDefinition was updated to require the
engine to be passed in, but it seems that update was missed here.

BackupDumper::loadPlugin() requires the second argument, set it to
the empty string to keep current behaviour.

Change-Id: Ifbd8fc4870ff63b2d338f8bb4d251d7a3477b989