Verify if fallback is in place for event_payload/eventAttributes in .NET SDK

Problem

During the rollout of dogfooding backend internal events to Product Analytics, we had events violating the custom_event schema and the event failing enrichment, because the Product Analytics Ruby SDK doesn't validate what is passed as event_payload, which is then directly passed as event props to SnowplowTracker::Tracker. This can be nil or any object otherwise not coercible to a JSON object, which breaks the schema.

Problems in gitlab-com related to this were mitigated by gitlab-org/gitlab!139073 (merged), but the SDKs shouldn't let developers emit an obviously invalid event.

Desired Outcome

.NET SDK should work as expected even if event_payload is not passed.

Proposed Solution

Update test cases and verify if track and identify calls are working without event_payload.