refactoring: just return 0 if the data is empty for the average

src/average_median/average_median.c

@@ -4,11 +4,16 @@
 #include <math.h>
 
 double average(const double *data, int n) {
+    if (n==0) {
+        return 0;
+    }
+    else {
     double sum = 0.0;
     for (int i = 0; i < n; ++i) {
         sum += data[i];
     }
     return sum / n;
+    }
 }
 
 double median(double *data, int n) {