fix(torbiak#38): add a guard for slices with length 0

When the slice's length is 0, should directly return to prevent false
result or slice out of bounds runtime error

ex4.5/unique.go

@@ -2,6 +2,10 @@
 package unique
 
 func unique(strs []string) []string {
+	if len(strs) == 0 {
+		return strs
+	}
+
 	w := 0 // index of last written string
 	for _, s := range strs {
 		if strs[w] == s {

ex4.5/unique_test.go

@@ -6,10 +6,21 @@ import (
 )
 
 func TestUnique(t *testing.T) {
-	s := []string{"a", "a", "b", "c", "c", "c", "d", "d", "e"}
-	got := unique(s)
-	want := []string{"a", "b", "c", "d", "e"}
-	if !reflect.DeepEqual(got, want) {
-		t.Errorf("got %v, want %v", got, want)
+	ss := [][]string{
+		{"a", "a", "b", "c", "c", "c", "d", "d", "e"},
+		{},
 	}
+
+	want := [][]string{
+		{"a", "b", "c", "d", "e"},
+		{},
+	}
+
+	for i, s := range ss {
+		got := unique(s)
+		if !reflect.DeepEqual(got, want[i]) {
+			t.Errorf("got %v, want %v", got, want[i])
+		}
+	}
+
 }