Handle case where getMemo requests a non-existent key

[jeffschoner-stripe]

What was changed

This change causes Workflow.getMemo to return null when called with a key that is not present on the memo.

The existing testMemoInWorkflow test has been modified to test that a missing key passed to getMemo returns null. It also extends the test to cover when a memo is queried with a mismatched type.

A new unit test, testNoMemoInWorkflowFailsGetMemoGracefully, has been added to check the case where getMemo returns null for any key when no memo was specified upon starting a workflow.

Why?

Workflow.getMemo currently throws a NullPointerException when an unknown memo key is requested. Because there's not another way to check for a key's existence before calling getMemo, the only way to handle this is to catch NullPointerException. Checking for null is better for flow control.

Checklist

2. How was this tested:

cd temporal-sdk && ../gradlew test --tests io.temporal.workflow.MemoTest

[CLAassistant]

All committers have signed the CLA.

[Spikhalskiy]

Thank you for the contribution!