fix: crash when the arr argument for concatArr isn't specified

fix: formatting

Co-authored-by: Almeida <almeidx@pm.me>

backend/src/templateFormatter.ts

@@ -346,6 +346,7 @@ const baseValues = {
     return [...args].join("");
   },
   concatArr(arr, separator = "") {
+    if (!Array.isArray(arr)) return "";
     return arr.join(separator);
   },
   eq(...args) {