fix CopyStrings and ShuffleStrings for slice when slice is nil

Signed-off-by: allencloud <allen.sun@daocloud.io>

pkg/util/slice/slice_test.go

@@ -22,15 +22,29 @@ import (
 )
 
 func TestCopyStrings(t *testing.T) {
-	src := []string{"a", "c", "b"}
-	dest := CopyStrings(src)
+	var src1 []string
+	dest1 := CopyStrings(src1)
 
-	if !reflect.DeepEqual(src, dest) {
-		t.Errorf("%v and %v are not equal", src, dest)
+	if !reflect.DeepEqual(src1, dest1) {
+		t.Errorf("%v and %v are not equal", src1, dest1)
 	}
 
-	src[0] = "A"
-	if reflect.DeepEqual(src, dest) {
+	src2 := []string{}
+	dest2 := CopyStrings(src2)
+
+	if !reflect.DeepEqual(src2, dest2) {
+		t.Errorf("%v and %v are not equal", src2, dest2)
+	}
+
+	src3 := []string{"a", "c", "b"}
+	dest3 := CopyStrings(src3)
+
+	if !reflect.DeepEqual(src3, dest3) {
+		t.Errorf("%v and %v are not equal", src3, dest3)
+	}
+
+	src3[0] = "A"
+	if reflect.DeepEqual(src3, dest3) {
 		t.Errorf("CopyStrings didn't make a copy")
 	}
 }
@@ -50,9 +64,16 @@ func TestSortStrings(t *testing.T) {
 }
 
 func TestShuffleStrings(t *testing.T) {
-	src := []string{"a", "b", "c", "d", "e", "f"}
+	var src []string
 	dest := ShuffleStrings(src)
 
+	if dest != nil {
+		t.Errorf("ShuffleStrings for a nil slice got a non-nil slice")
+	}
+
+	src = []string{"a", "b", "c", "d", "e", "f"}
+	dest = ShuffleStrings(src)
+
 	if len(src) != len(dest) {
 		t.Errorf("Shuffled slice is wrong length, expected %v got %v", len(src), len(dest))
 	}