.Net: [MEVD] Stop silently ignoring null generated keys in upserts

[roji]

• In the batching version of some of our connectors, some providers filter out null values, while others don't.
• Note that the corresponding non-batching UpsertAsync does not filter out nulls, which is inconsistent behavior.
• It's important for the list of keys returned from UpsertAsync to correspond exactly - position-wise - with the records passed in. The main point of returning those keys is that they can be correlated back to their records, so that those records can be fetched again later. If we silently filter out a null key, the wrong key would get correlated to the wrong record.
• In any case, I don't believe there's legitimate scenarios where a key would be null (database-generated or not), so this feels like it should be an error rather than for them to get silently filtered out.

@westey-m @dmytrostruk am I missing something here, was there some interesting behavior in some of the connectors which requires null key filtering?

/cc @adamsitnik

(note that if we do something here, it should be done after #10692 (which I'm currently implemented))