don't emit a PHP warning on a null string with 'use label as value'

JonGold requested to merge JonGold/dataprocessor:null-strings into master

My logs were full of this warning:

Warning: Invalid argument supplied for foreach() in Civi\DataProcessor\FieldOutputHandler\ArrayFieldOutputHandler->formatField() (line 78 of /mysite/web/sites/all/civicrm-custom/extensions/dataprocessor/Civi/DataProcessor/FieldOutputHandler/ArrayFieldOutputHandler.php) 

This ensures that the value being converted isn't NULL to avoid this warning.

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