Fix bugs with entrypoint/command in docker-compose run

- When no command is passed but `--entrypoint` is, set Cmd to `[]`
- When command is a single empty string, set Cmd to `[""]`

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

tests/fixtures/entrypoint-dockerfile/Dockerfile

@@ -0,0 +1,4 @@
+FROM busybox:latest
+LABEL com.docker.compose.test_image=true
+ENTRYPOINT ["printf"]
+CMD ["default", "args"]