Hide symptoms of unset placeholders in data[0]

Missing placeholders when Merge Rows and Reduce Data were causing php
warnings to appear e.g.

Notice: Undefined property: stdClass::$fch_cat___id
in/homepages/3/d85014684/htdocs/fairmile/dev/joomla3/components/com_fabrik/models/list.php
on line 9455

This fix checks that the property exists as well as providing some
optimised property lookups.