fix(java-sdk): custom properties patch client

[shirshanka]

• Fixes an issue with the custom properties patch client in Java where if custom properties are the only things being changed, the parent patcher assumes there are no patches and throws an exception on build().
• Changes the interaction model between the parent patch builder and the inner builder. The inner builder now directly adds to the parent's "operations list".

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub