[GOBBLIN-2169] Fix NPE while overwrite if table has no previous 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):
  While overwrite we log the snapshot id of last snapshot present before commit but in case of table with no snapshot (possible freshly created table) currentSnapshot will be null hence will give NPE if used to fetch SnapshotId
  This PR fixes that with proper logging.

Tests

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

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"