Ignore polygons with length 0 when updating disallowed areas
Ignore polygons with length 0 when updating disallowed areas

Contributes to CURA-3148

ynotstartups/Wanhao,hmflash/Cura,Curahelper/Cura,Curahelper/Cura,hmflash/Cura,ynotstartups/Wanhao,fieldOfView/Cura,fieldOfView/Cura