fix(browseDAO): Handle null browse path from ES in BrowseDAO

[pinakipb2]

Checklist

• [x] The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• [ ] Links to related issues (if applicable)
• [x] 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

Background:

In PR: https://github.com/datahub-project/datahub/pull/11514/files, empty strings were considered as null for ES indexing.

In Kibana:

From DB (aspect):

Issue:

This is causing null pointer exception for entities, where browse path is empty string.

Which is emulated in the below test:

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Have added test for the changes as well.