fix: don't overwrite with empty cmd/entrypoint (#1076)

Co-authored-by: ChristopherHX <christopher.homberger@web.de>

Signed-off-by: Ryan <me@hackerc.at>

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

pkg/runner/testdata/uses-docker-url/push.yml

@@ -9,3 +9,15 @@ jobs:
         with:
           somekey: somevalue
           args: echo ${INPUT_SOMEKEY} | grep somevalue
+      - uses: docker://node:16-buster-slim
+        with:
+          args: -v
+      - uses: docker://node:16-buster-slim
+        with:
+          entrypoint: /bin/sh
+          args: -c -- uname -a
+      - uses: docker://node:16-buster-slim
+        with:
+          entrypoint: /bin/sh
+          args: -c 'uname -a'
+