Insert guard clause for workflow.upsert_search_attributes on empty inputs

[accupham]

Fix for #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

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

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[cretz]

Thanks! May need to sign CLA.

[accupham]

@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.