What changes were proposed in this pull request?

Fix three NPE issues reported by Sonar Qube in the server impl.

• Add default to streamEndOfRequestAsync function
• Throw error on notifyStateMachine... funtion
• Remove possibility for new cache entries to be null
• Add tests for changes

Was part of the #461

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/RATIS-1368

How was this patch tested?

Changes have been tested using existing test cases and two new test cases.