`EmailAttribute` should allow null values

[alexrp]

Specifically:

https://github.com/jstedfast/EmailValidation/blob/643a8e3cf3dd93f5eba60e27639728df8d3457a3/EmailValidation/EmailAttribute.cs#L86-L87

https://github.com/jstedfast/EmailValidation/blob/643a8e3cf3dd93f5eba60e27639728df8d3457a3/EmailValidation/EmailAttribute.cs#L105

---

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.