fix(graphql): Skip schema fields with empty fieldPaths to prevent the dataset mapper from erroring out

[jayasimhankv]

Background:
There have been some cases wherein the Schema fieldPath is ingested as an empty string due to which the UI retrieval (or the GQL query) fails in the SchemaMapper. Although the validation on field path has now been introduced with v.0.15 which would prevent schema fields with empty fieldPaths from being ingested, (ref code commit: https://github.com/datahub-project/datahub/blob/master/entity-registry/src/main/java/com/linkedin/metadata/aspect/validation/FieldPathValidator.java#L112), it would be ideal to add a check to eliminate fields with empty fieldPaths before mapping them so such existing datasets could be retrieved. This PR attempts to do so.

Example schema field with empty field path (data ingested pre-v0.15 FieldPathValidator implementation)

Failure retrieving this dataset:

GetDataset call fails:

Mapper error:
java.lang.IllegalArgumentException: Cannot create a Urn from tuple with an empty value

UI works after the fix and eliminates the field with empty fieldPath:

@rtekal - FYI please.

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

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All modified and coverable lines are covered by tests ✅

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[rtekal]

This PR looks good. Adds checks while retrieving data.