Nullability Annotations

[tanveerbadar]

CsvHelper doesn't have nullability annotations.

Apply nullability annotations to various types as appropriate.

I cannot rely on this great library supporting compiler's nullability analysis at this point in time.

Take CsvReader.ReadHeader() for instance. It should have an annotation of [MemberNotNullWhen(true, nameof(HeaderRecord))]. Otherwise, users are forced to check the return value and header not being null for shutting up the compiler.