Reading or setting a null flag should be considered invalid

### I am planning on implementing this myself

Yes

### Describe your request

Flags are always assumed to be non-`null` strings, even though `null` flags are technically valid. As such, reading or setting a `null` flag should in my opinion be considered a bug in the caller.

Everest should no longer support reading or setting a `null` flag and instead throw an `ArgumentNullException` in this situation.

### Additional context

_No response_