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 <[email protected]>
Co-authored-by: silverwind <[email protected]>

act/runner/runner_test.go

@@ -303,6 +303,7 @@ func TestRunEvent(t *testing.T) {
 		// services
 		{workdir, "services", "push", "", platforms, secrets},
 		{workdir, "services-with-container", "push", "", platforms, secrets},
+		{workdir, "services-empty-image", "push", "", platforms, secrets},
 
 		// local remote action overrides
 		{workdir, "local-remote-action-overrides", "push", "", platforms, secrets},