invalid null.Time will MarshalText to an empty string

this aligns with behavior of the other null types
previously it marshaled to the string "null"
fixes #47

time.go

@@ -110,15 +110,17 @@ func (t *Time) UnmarshalJSON(data []byte) error {
 	return err
 }
 
+// MarshalText returns an empty string if invalid, otherwise time.Time's MarshalText.
 func (t Time) MarshalText() ([]byte, error) {
 	if !t.Valid {
-		return []byte("null"), nil
+		return []byte{}, nil
 	}
 	return t.Time.MarshalText()
 }
 
 func (t *Time) UnmarshalText(text []byte) error {
 	str := string(text)
+	// allowing "null" is for backwards compatibility with v3
 	if str == "" || str == "null" {
 		t.Valid = false
 		return nil

time_test.go

@@ -80,7 +80,7 @@ func TestUnmarshalTimeText(t *testing.T) {
 	assertNullTime(t, null, "unmarshal null text")
 	txt, err = null.MarshalText()
 	maybePanic(err)
-	assertJSONEquals(t, txt, string(nullJSON), "marshal null text")
+	assertJSONEquals(t, txt, "", "marshal null text")
 
 	var invalid Time
 	err = invalid.UnmarshalText([]byte("hello world"))

zero/time.go

@@ -117,6 +117,8 @@ func (t *Time) UnmarshalJSON(data []byte) error {
 	}
 }
 
+// MarshalText implements encoding.TextMarshaler.
+// It will encode to an empty time.Time if invalid.
 func (t Time) MarshalText() ([]byte, error) {
 	ti := t.Time
 	if !t.Valid {
@@ -125,8 +127,12 @@ func (t Time) MarshalText() ([]byte, error) {
 	return ti.MarshalText()
 }
 
+// UnmarshalText implements encoding.TextUnmarshaler.
+// It has compatibility with the null package in that it will accept empty strings as invalid values,
+// which will be unmarshaled to a zero value.
 func (t *Time) UnmarshalText(text []byte) error {
 	str := string(text)
+	// allowing "null" is for backwards compatibility with v3
 	if str == "" || str == "null" {
 		t.Valid = false
 		return nil