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/dockerfile_with_entrypoint/docker-compose.yml

@@ -1,2 +0,0 @@
-service:
-  build: .

tests/fixtures/entrypoint-composefile/docker-compose.yml

@@ -0,0 +1,6 @@
+version: "2"
+services:
+  test:
+    image: busybox
+    entrypoint: printf
+    command: default args

tests/fixtures/entrypoint-dockerfile/Dockerfile

@@ -1,3 +1,4 @@
 FROM busybox:latest
 LABEL com.docker.compose.test_image=true
-ENTRYPOINT echo "From prebuilt entrypoint"
+ENTRYPOINT ["printf"]
+CMD ["default", "args"]

tests/fixtures/entrypoint-dockerfile/docker-compose.yml

@@ -0,0 +1,4 @@
+version: "2"
+services:
+  test:
+    build: .