fix: skip service containers with an empty image (#1074)

GitHub Actions skips services whose image evaluates to an empty string, enabling conditional services via expressions like `image: ${{ matrix.image || '' }}`. Here such a service failed the job at `docker create`. Skip them before container creation, using GitHub's log message verbatim, and add a regression test.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1074
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

act/runner/testdata/services-empty-image/push.yml

@@ -0,0 +1,10 @@
+name: services-empty-image
+on: push
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    services:
+      db:
+        image: ${{ false && 'postgres:16' || '' }}
+    steps:
+      - run: echo "empty-image service was skipped"