[GOBBLIN-2169] Fix NPE 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):
  NPE can occur if table has no previous snapshot to copy data files from. My changes aim to fix this and throw io exception incase no snapshot found for copying in place of unexpected NPE.

Tests

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

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"