fix(graph-edge): fix graph edge delete exception

[david-leifker]

Fixing an exception that occurs when deleting graph edges when the MCL is missing the record template. Code updated to check the previous aspect and the current aspect during a delete.

Additionally, if both aspects are missing avoid an exception and log a warning.

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

[esteban]

Why can we end up with a null aspect? Seems that some of the implementations from RecordTemplate can be either Nullable and Nonnull Just curious if might be just better to return an empty record template that handling a mix of Nullable and Nonnull that could end up in NPEs.

[david-leifker]

The MCL usually contains both a current aspect and a previous aspect, the previous aspect is null when the aspect is being first created. For a delete, it doesn't make sense that the current aspect would be populated necessarily. I would expect a previous aspect unless we hit a third case which is a delete of the key aspect of an entity which should technically be added separately but is complete redundant with the entity urn.

In any case, we need to respect that the data model defined both current and previous as optional here and here. Changing the model is a backwards incompatible change.