C#: Fix some new compiler warnings

[michaelnebel]

A couple of new compiler warnings (since Friday)

• A compiler warning for MessagePack 3.0.3 (I think the package was just renamed to 3.0.300, which is not flagged as having a security vulnerability).
• Possible null de-reference warnings (fixed by adding some null checks).

DCA looks good.

[tamasvajk]

Should we log something in this case? Do we expect any of these to be null in any circumstance?

[michaelnebel]

Good question, I am not really familiar with this API. This was the minimal change I could make that just replaced a potential crash with a failed validation instead.
@mbg : Do you have an opinion about it?

[mbg]

No opinion. I couldn't see anything obvious in the docs about when these might be null and they were never null during testing. I didn't implement any checks for whether these are null, since I figured that would be more helpful to know under what circumstances they end up being null than just return false.

[michaelnebel]

Ok, that indicates that we should add a log message in case these are null.

[mbg]

Yep, I was just about to add that I guess logging it as @tamasvajk suggested would make sense.

[mbg]

Thank you for addressing those details!