Fix `encodeBase64` for empty password or user in HTTP Basic Authentication (#4326)

server/model/monitor.js

@@ -230,10 +230,12 @@ class Monitor extends BeanModel {
     /**
      * Encode user and password to Base64 encoding
      * for HTTP "basic" auth, as per RFC-7617
+     * @param {string|null} user - The username (nullable if not changed by a user)
+     * @param {string|null} pass - The password (nullable if not changed by a user)
      * @returns {string}
      */
     encodeBase64(user, pass) {
-        return Buffer.from(user + ":" + pass).toString("base64");
+        return Buffer.from(`${user || ""}:${pass || ""}`).toString("base64");
     }
 
     /**