Don't log spurious error when extra is empty

[jeremybeard]

Description

This fixes a spurious error log where if a connection is loaded into the Airflow DB, and the extra field on the connection is empty, it prints:

Error converting extra to map for my_conn_id, found type: <nil>

This does not block the connection from loading, but it is confusing, so this change removes it.

🧪 Functional Testing

• Added unit test
• Manually checked that the spurious error no longer appears

📸 Screenshots

Before:

After:

📋 Checklist

• [x] Rebased from the main (or release if patching) branch (before testing)
• [x] Ran make test before taking out of draft
• [x] Ran make lint before taking out of draft
• [x] Added/updated applicable tests
• [x] Tested against Astro-API (if necessary).
• [ ] Tested against Houston-API and Astronomer (if necessary).
• [ ] Communicated to/tagged owners of respective clients potentially impacted by these changes.
• [ ] Updated any related documentation

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.76%. Comparing base (5e0cdac) to head (2398720).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1648 +/- ## ======================================= Coverage 86.76% 86.76% ======================================= Files 114 114 Lines 16693 16695 +2 ======================================= + Hits 14483 14485 +2 Misses 1323 1323 Partials 887 887 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.