Fix panic when printing a nil Date (#419)

* fix: fix panic when printing the nil Date

Not sure why, but this is only happening when running with delve.
It seems like printing a nil Stringer causes a panic when debugging
with delve but is handled gracefully outside of delve.

Steps to reproduce:
1. Start debugger
2. Print the value of a nil Date
```go
var d *Date
fmt.Sprintf("%v", d)
```

Theory:
`time.Time` is embedded within the `Date` type. This means that if
Date is nil and Stringer is used to get the string representation of
it, it'll try to call Date.Time.String() which will result in a nil
dereferencing error.

* Regenerate code

Co-authored-by: Beng Lim <beng.lim@outlook.com>

Author: autobotbengg

date.go

@@ -28,3 +28,7 @@ func (d *Date) UnmarshalJSON(data []byte) error {
 	d.Time = parsed
 	return nil
 }
+
+func (d Date) String() string {
+	return d.Time.Format(DateFormat)
+}

date_test.go

@@ -2,6 +2,7 @@ package types
 
 import (
 	"encoding/json"
+	"fmt"
 	"testing"
 	"time"
 
@@ -30,3 +31,24 @@ func TestDate_UnmarshalJSON(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, testDate, b.DateField.Time)
 }
+
+func TestDate_Stringer(t *testing.T) {
+	t.Run("nil date", func(t *testing.T) {
+		var d *Date
+		assert.Equal(t, "<nil>", fmt.Sprintf("%v", d))
+	})
+
+	t.Run("ptr date", func(t *testing.T) {
+		d := &Date{
+			Time: time.Date(2019, 4, 1, 0, 0, 0, 0, time.UTC),
+		}
+		assert.Equal(t, "2019-04-01", fmt.Sprintf("%v", d))
+	})
+
+	t.Run("value date", func(t *testing.T) {
+		d := Date{
+			Time: time.Date(2019, 4, 1, 0, 0, 0, 0, time.UTC),
+		}
+		assert.Equal(t, "2019-04-01", fmt.Sprintf("%v", d))
+	})
+}