cmd: Only validate config is proper JSON if config slice has data (#6250)

* cmd: fix error when running without config

* ci: add smoke test

.github/workflows/ci.yml

@@ -101,6 +101,12 @@ jobs:
       run: |
         go build -tags nobdger -trimpath -ldflags="-w -s" -v
 
+    - name: Smoke test Caddy
+      working-directory: ./cmd/caddy
+      run: |
+        ./caddy start
+        ./caddy stop
+
     - name: Publish Build Artifact
       uses: actions/upload-artifact@v4
       with:

cmd/main.go

@@ -199,7 +199,7 @@ func loadConfigWithLogger(logger *zap.Logger, configFile, adapterName string) ([
 				zap.Int("line", warn.Line))
 		}
 		config = adaptedConfig
-	} else {
+	} else if len(config) != 0 {
 		// validate that the config is at least valid JSON
 		err = json.Unmarshal(config, new(any))
 		if err != nil {