Additional safety for head check

.eleventy.js

@@ -29,6 +29,9 @@ module.exports = function(eleventyConfig) {
 
   // Get the first `n` elements of a collection.
   eleventyConfig.addFilter("head", (array, n) => {
+    if(!Array.isArray(array) || array.length === 0) {
+      return [];
+    }
     if( n < 0 ) {
       return array.slice(n);
     }