EmailAttribute should allow null values

[alexrp]

Specifically:

EmailValidation/EmailValidation/EmailAttribute.cs

Lines 86 to 87 in 643a8e3

	if (value == null)
	return new ValidationResult ("Email can't be null", memberNames);

EmailValidation/EmailValidation/EmailAttribute.cs

Line 105 in 643a8e3

return value != null && EmailValidator.Validate ((string) value, AllowTopLevelDomains, AllowInternational);

---

Most (all?) attributes in System.ComponentModel.DataAnnotations of a similar nature to EmailAttribute consider null to be valid.

Examples:

• https://github.com/dotnet/runtime/blob/88868b7a781f4e5b9037b8721f30440207a7aa42/src/libraries/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/PhoneAttribute.cs#L23-L28
• https://github.com/dotnet/runtime/blob/88868b7a781f4e5b9037b8721f30440207a7aa42/src/libraries/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/CreditCardAttribute.cs#L18-L23
• https://github.com/dotnet/runtime/blob/88868b7a781f4e5b9037b8721f30440207a7aa42/src/libraries/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/RegularExpressionAttribute.cs#L63-L67

Generally speaking, one should use RequiredAttribute if a value should not be allowed to be null. (Higher-level frameworks like ASP.NET Core will additionally use C# nullability annotations to infer RequiredAttribute.)

[alexrp]

(Aside: The allocation of memberNames is pointless in the success case.)

[jstedfast]

Seems reasonable. Hopefully won't break any existing apps due to a change in expectations.