caddy.go: add oldCfg length check,fix index write error. (#6991)

Author: Chise1

caddy.go

@@ -265,9 +265,16 @@ func changeConfig(method, path string, input []byte, ifMatchHeader string, force
 
 func rollbackRawCfg() error {
 	var oldCfg any
-	err := json.Unmarshal(rawCfgJSON, &oldCfg)
-	rawCfg[rawConfigKey] = oldCfg
-	return err
+	if len(rawCfgJSON) != 0 {
+		err := json.Unmarshal(rawCfgJSON, &oldCfg)
+		if err != nil {
+			return err
+		}
+		rawCfg[rawConfigKey] = oldCfg
+	} else {
+		rawCfg[rawConfigKey] = nil
+	}
+	return nil
 }
 
 // readConfig traverses the current config to path
@@ -297,7 +304,7 @@ func indexConfigObjects(ptr any, configPath string, index map[string]string) err
 			if k == idKey {
 				switch idVal := v.(type) {
 				case string:
-					err := writeToIdIndex(index, idVal, k)
+					err := writeToIdIndex(index, idVal, configPath)
 					if err != nil {
 						return err
 					}