[Bug] Workflow codec not working with new timer summary

[cretz]

Describe the bug

At

sdk-dotnet/src/Temporalio/Worker/WorkflowCodecHelper.cs

Line 196 in b5ca854

await EncodeAsync(codec, cmd.StartTimer.Summary).ConfigureAwait(false);

, we assume the summary is non-null which is not correct. We need to fix this and we need to ensure coverage.