Make TagSet.Remove a no-op when contentType is nil

Previously, syncTagSet.Remove() can panic if function parameter
contentType is nil.

This commit adds an early return so Remove(nil) is a silent no-op,
mirroring the behavior of Go's built-in delete() on a missing key.

Author: fxamacker

tag.go

@@ -155,6 +155,7 @@ type TagSet interface {
 	Add(opts TagOptions, contentType reflect.Type, num uint64, nestedNum ...uint64) error
 
 	// Remove removes given tag content type from TagSet.
+	// Remove is a no-op if contentType is nil.
 	Remove(contentType reflect.Type)
 
 	tagProvider
@@ -245,7 +246,11 @@ func (t *syncTagSet) Add(opts TagOptions, contentType reflect.Type, num uint64,
 }
 
 // Remove removes given tag content type from TagSet.
+// Remove is a no-op if contentType is nil.
 func (t *syncTagSet) Remove(contentType reflect.Type) {
+	if contentType == nil {
+		return
+	}
 	for contentType.Kind() == reflect.Pointer {
 		contentType = contentType.Elem()
 	}

tag_test.go

@@ -569,6 +569,44 @@ func TestAddRemoveTag(t *testing.T) {
 	tags.Remove(myFloatType)
 }
 
+func TestTagSetRemoveNil(t *testing.T) {
+	type myInt int
+	myIntType := reflect.TypeOf(myInt(0))
+
+	tags := NewTagSet()
+	stags := tags.(*syncTagSet)
+
+	// Remove nil contentType from an empty TagSet
+	tags.Remove(nil)
+	if len(stags.t) != 0 {
+		t.Errorf("TagSet len is %d, want %d", len(stags.t), 0)
+	}
+
+	if err := tags.Add(TagOptions{DecTag: DecTagRequired, EncTag: EncTagRequired}, myIntType, 100); err != nil {
+		t.Errorf("TagSet.Add(%s, %d) returned error %v", myIntType.String(), 100, err)
+	}
+	if len(stags.t) != 1 {
+		t.Errorf("TagSet len is %d, want %d", len(stags.t), 1)
+	}
+
+	// Remove nil contentType from non-empty TagSet
+	tags.Remove(nil)
+	if len(stags.t) != 1 {
+		t.Errorf("TagSet len is %d, want %d", len(stags.t), 1)
+	}
+
+	expectedTI := &tagItem{
+		num:         []uint64{100},
+		cborTagNum:  []byte{0xd8, 0x64},
+		contentType: myIntType,
+		opts:        TagOptions{DecTag: DecTagRequired, EncTag: EncTagRequired},
+	}
+	ti := tags.getTagItemFromType(myIntType)
+	if !reflect.DeepEqual(ti, expectedTI) {
+		t.Errorf("tagItem is %+v, want %+v", ti, expectedTI)
+	}
+}
+
 func TestAddTagTypeAliasError(t *testing.T) {
 	type myBool = bool
 	type myUint = uint