refactoring: return 0 for the median if it is empty

src/average_median/average_median.c

@@ -20,7 +20,10 @@ double average(const double *data, int n) {
 }
 
 double median(double *data, int n) {
-    if (n % 2 == 0) {
+      if (n==0) {
+        return 0;
+    }
+    else if (n % 2 == 0) {
         return (data[n / 2 - 1] + data[n / 2]) / 2.0;
     } else {
         return data[n / 2];