refactoring: if one a or b is zero just return the other

src/subtraction/subtraction.c

@@ -1,3 +1,6 @@
+#include "subtraction.h"
 int subtract(int a, int b) {
-    return a - b;
+    if (a==0) return b;
+    if(b==0) return a;
+    else return a - b;
 }