Fix assignee being cleared when upstream user has no fullname

[Bala-Sakabattula]

Summary

• Fix a bug in _update_assignee() where an upstream GitHub assignee without a fullname in their profile would cause the downstream Jira assignee to be incorrectly cleared. The root cause was that us_exists checked for fullname (optional) instead of login (always present), so a valid assignee with no display name was treated as "no assignee", triggering a de-assignment.
• When login exists but fullname is absent, assign_user() is now called to resolve the user via LDAP/email lookup rather than clearing the Jira assignee.
• Expanded test_update_assignee_all with a new upstream variant (login present, no fullname key) and updated existing test data to include login fields, reflecting the corrected us_exists semantics.

[Bala-Sakabattula]

@webbnh I made the changes as per suggestion.. can you kindly please review again??

[webbnh]

👍