GetStreamMetadataAsync returns a null Tags dictionary, but StreamMetadata.Tags is declared non-nullable

[jeremydmiller]

Found by the new EventStoreExplorerCompliance suite (JasperFx.Events.ComplianceTests, compliance wave 5 — marten#5146).

IEventStore.GetStreamMetadataAsync(streamId, ct) returns a StreamMetadata whose Tags property is null.

JasperFx.Descriptors.StreamMetadata declares it as a non-nullable positional parameter:

public sealed record StreamMetadata(
    ...
    IReadOnlyDictionary<string, string> Tags);

So null is a contract violation rather than a legitimate "this stream has no tags" answer — an empty dictionary is how that is spelled. Any consumer that trusts the declaration and enumerates Tags gets a NullReferenceException instead of an empty loop. Marten returns a non-null dictionary here.

Repro

var metadata = await store.As<IEventStore>().GetStreamMetadataAsync(streamId.ToString(), ct);
metadata.Tags.ShouldNotBeNull(); // fails on Polecat

Current state

Gated in the compliance suite behind EventStoreComplianceFixture.SupportsStreamMetadataTags, which PolecatComplianceFixture overrides to false with a pointer to this issue. The gate is temporary and tracked — the assertion stays.

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