Update pair_sum_unsorted.go

Use `return []int{}` instead of `return nil` in order to keep consistency with origial Python solition.

Author: ineBallardin

go/Hash Maps and Sets/pair_sum_unsorted.go

@@ -7,5 +7,5 @@ func pairSumUnsorted(nums []int, target int) []int {
 		}
 		hashmap[x] = i
 	}
-	return nil
+	return []int{}
 }