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.

See http://fabrikar.com/forums/index.php?threads/front-end-list-back-end-list-process-differently.34789/ for debugging trail. Root cause turned out to be a mixture of security settings and setting some but not all repeating fields Include in List Query to No.

Bottom line on debugging: I think we should probably include this fix, and code another fix to:

1. At run-time, override primary / foreign key "Include in List Query" to yes if other fields in the repeat group are set to yes.