Make getEmptyInstanceContainer return a container with ID 'empty'
Make getEmptyInstanceContainer return a container with ID 'empty'

This is checked in the test. Otherwise it would return magic mocks.

Contributes to issue CURA-3497.

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