Fix for undefined source in Vector layer
Fix for undefined source in Vector layer

A source left or set undefined is equivalent to an empty source so treat it as such to prevent errors in prepareFrame
ahocevar/openlayers,geekdenz/openlayers,stweil/ol3,geekdenz/openlayers,geekdenz/ol3,oterral/ol3,openlayers/openlayers,geekdenz/ol3,adube/ol3,geekdenz/ol3,openlayers/openlayers,stweil/ol3,oterral/ol3,ahocevar/ol3,ahocevar/ol3,ahocevar/openlayers,bjornharrtell/ol3,geekdenz/ol3,ahocevar/ol3,adube/ol3,stweil/openlayers,openlayers/openlayers,ahocevar/ol3,ahocevar/openlayers,adube/ol3,bjornharrtell/ol3,oterral/ol3,stweil/ol3,geekdenz/openlayers,stweil/openlayers,stweil/ol3,stweil/openlayers,bjornharrtell/ol3