Fixes #512.

As described in the issue, allowing null for Value breaks the assumed contract and leads to various inconsistencies.
On top of that, the internal implementation assumes the Value won't be null. The equality operators, hash code, implicit/explicit operators, all of them are based on the premise that the Value is not null.

If the consumers need null (e.g. database columns), then the whole thing can be null, e.g. MyEnum?. But, once you have an instance of the enum, we should guarantee a non-null Value field.

The PR #412 was merged and published as part of version 8 (not so long ago). If needed we can keep this PR open, until a larger consensus is reached on this topic.

PS. The Name indeed can be empty, or eventually null. We may add support for that I assume. Actually, that was the original request in this issue #302.