[GOBBLIN-496] Support nullable union when getting field schema

[jack-moseley]

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-496

Description

• Here are some details about my PR, including screenshots (if applicable):

AvroUtils.getFieldSchema currently throws an error if there is any complex union. This change allows getting the schema if there is a union of complex and null types.

Tests

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

Added unit test for this case.

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"

[jack-moseley]

@htran1 could you review?

[htran1]

+1.