Check for no `Cmd` on exec create endpoint

Fixes #9414

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

Docker-DCO-1.1-Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: jfrazelle)
Bump version to v1.4.0

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)