Fix warning when no gateway groups
Fix warning when no gateway groups

If there are no gateway groups defined, and you save a rule that has an ordinary gateway selected in "Advanced Features - Gateway", then a warning is emitted when trying to traverse an empty gateway groups array at line 214.
brunostein/pfsense,phil-davis/pfsense,phil-davis/pfsense,phil-davis/pfsense,jxmx/pfsense,brunostein/pfsense,BlackstarGroup/pfsense,NewEraCracker/pfsense,ptorsten/pfsense,BlackstarGroup/pfsense,pfsense/pfsense,ch1c4um/pfsense,NOYB/pfsense,NewEraCracker/pfsense,BlackstarGroup/pfsense,pfsense/pfsense,dennypage/pfsense,pfsense/pfsense,ptorsten/pfsense,ptorsten/pfsense,ch1c4um/pfsense,ch1c4um/pfsense,dennypage/pfsense,NewEraCracker/pfsense,ch1c4um/pfsense,NOYB/pfsense,jxmx/pfsense,NOYB/pfsense,jxmx/pfsense,brunostein/pfsense,BlackstarGroup/pfsense,NewEraCracker/pfsense,dennypage/pfsense,dennypage/pfsense,pfsense/pfsense,phil-davis/pfsense