fix: leave JSON nulls alone

Author: enisdenjo

gorillamux/gorillamux.go

@@ -190,8 +190,7 @@ func jsonBodyFieldsAsVars(body []byte, mapping map[string]string, vars map[strin
 		valueStr := string(value)
 		switch {
 		case valueStr == "null":
-			// JSON null is handled as empty string command arg
-			vars[name] = ""
+			// JSON nulls are left alone
 
 		case valueStr[0] == '"':
 			// Unescape JSON string