Adds support for 'Nil' and 'Empty' rules

[samber]

• Nil is valid when a pointer is nil
• Empty is valid when non-nil pointers is empty (== valid when nil || empty)

Example:

type People struct { DeprecatedField *string Type string IDCard string } func (p People) Validate() error { return validation.ValidateStruct(&p, validation.Field(&p.DeprecatedField, validation.Nil), validation.Field(&p.IDCard, validation.When(a.Type != "citizen", validation.Empty)),	) } func main() { str := "hello world" p := People{ DeprecatedField: &str, Type: "foreigner", IDCard: "1234567890",	} err := p.Validate() }

[coveralls]

Coverage remained the same at 100.0% when pulling bef11e1 on samber:feat-absent into f6ac6c4 on go-ozzo:master.

[qiangxue]

Thank you for your contribution!