also treat DOCKER_CONTEXT='' as unset
also treat DOCKER_CONTEXT='' as unset

Signed-off-by: Nick Santos <0900b535159286fa9dedbf866b62a8251f58595f@docker.com>

cpuguy83/cli,docker/cli,cpuguy83/cli,thaJeztah/cli,docker/cli,thaJeztah/cli