test: add validation for non-empty array in calculateMean function

Author: MattCarolus10

Sprint-1/implement/mean.js

@@ -2,6 +2,9 @@ function calculateMean(list) {
   if (!Array.isArray(list)) {
     throw new Error("calculateMean requires an array of numbers");
   }
+  if (list.length === 0) {
+    throw new Error("calculateMean requires a non-empty array");
+  }
 }
 
 module.exports = calculateMean;