fix parse_owners()

[LavMatt]

parse_owners was erroring with a pydantic validation error because displayName was not being set to fullName because it did actually exist with a value of None. This fixes that scenario

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: lib/datahub-client/data_platform_catalogue/client/graphql_helpers.py

Function	Unhandled Issue
parse_owner	ValidationError: 1 validation error for OwnerRef ... Event Count: 12

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