e2e: Use icmd.None in a couple more places
e2e: Use icmd.None in a couple more places

This checks for actual emptiness, while `""` means "don't care".

Signed-off-by: Ian Campbell <a8db091655a7f47ecb9c39a2d278e9662e440dc0@docker.com>

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