AVRO-3038: enable support for nullable array/enum

[Gonyoda]

Jira

• My PR addresses the following Avro Jira issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AVRO-3038

Tests

• My PR adds the following unit tests:
• NullableArrayTest
• NullableEnumTest

Commits

• My commits all reference Jira issues in their subject lines. 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 (not including Jira issue reference)
  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"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain Javadoc that explain what it does