DynamoDBMasterMonitor: Don't publish MASTER_NULL on Dynamo failure

[crioux-stripe]

Context

We encountered an issue in which DynamoDBMasterMonitor would emit a MASTER_NULL value in the event of failing to read or decode the lock from DynamoDB. This caused some minor but unncessary churn as our agents believed the master had changed when it in fact had not. This can be further exacerbated by transient failures in Dynamo or throttling when querying Dynamo.

Checklist

• [x] ./gradlew build compiles code correctly
• [x] Added new tests where applicable
• [x] ./gradlew test passes all tests
• [x] Extended README or added javadocs where applicable

[github-actions[bot]]

Test Results

535 tests  ±0   529 :white_check_mark: ±0   7m 51s :stopwatch: +8s 139 suites ±0     6 :zzz: ±0  139 files   ±0     0 :x: ±0 

Results for commit b3bba8c6. ± Comparison against base commit a61cf1dd.

:recycle: This comment has been updated with latest results.

[crioux-stripe]

Just realized I have a commented out test here. Working on a fix.

[crioux-stripe]

Fixed.