fix(MessageEmbed): avoid throwing error when accessing colorless hexColor

src/structures/MessageEmbed.js

@@ -113,10 +113,11 @@ class MessageEmbed {
 
   /**
    * The hexadecimal version of the embed color, with a leading hash
-   * @type {string}
+   * @type {?string}
    * @readonly
    */
   get hexColor() {
+    if (!this.color) return null;
     let col = this.color.toString(16);
     while (col.length < 6) col = `0${col}`;
     return `#${col}`;