Insert guard clause for workflow.upsert_search_attributes on empty inputs

[irdbl]

Fix for https://github.com/temporalio/sdk-python/issues/629

What was changed

Guard clause on workflow.upsert_search_attributes so SDK doesn't crash on empty input.

Why?

Spent 5 hours debugging an inscrutable error message.

Checklist

1. Closes 629

2. How was this tested: Pass empty list into upsert_search_attributes in a workflow.

3. Any docs updates needed? No

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[irdbl]

@cretz Signed CLA!

[cretz]

Thanks! We have a CI failure that is unrelated to this PR. Once it is fixed, we can come back and merge this.