[GOBBLIN-2169] Fix NPE when table has no snapshot while accessing current snapshot

[Blazer-007]

Dear Gobblin maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• [✔️] My PR addresses the following Gobblin JIRA issues and references them in the PR title. For example, "[GOBBLIN-XXX] My Gobblin PR"
  • https://issues.apache.org/jira/browse/GOBBLIN-2169

Description

• [✔️] Here are some details about my PR, including screenshots (if applicable):
  • NPE can occur if table has no previous snapshot while accessing current snapshot from tablemetadata.
  • Added NoSnapshotFoundException

Tests

• [✔️] My PR adds the following unit tests OR does not need testing for this extremely good reason:
  • testGetCurrentSnapshotInfoOnEmptyTable

Commits

• [✔️] My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"