Fix secret environment variables being updated with empty string

[vandyliu]

Description

Fix secret environment variables being updated. Nil values for existing secret env vars will no longer update the secret variables but keep them with the same value.

🎟 Issue(s)

https://github.com/astronomer/astro-cli/issues/1593

🧪 Functional Testing

• Added/updated unit tests

  List the functional testing steps to confirm this feature or fix.

📸 Screenshots

Updated the secret variable via ./astro deployment update <deploymentId> --deployment-file deployment.yaml

Saw on airflow UI that the env var did not get set back to ""

Add screenshots to illustrate the validity of these changes.

📋 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 86.56%. Comparing base (bf3ec8f) to head (5960ad1). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1618 +/- ## ======================================= Coverage 86.56% 86.56% ======================================= Files 114 114 Lines 16087 16087 ======================================= Hits 13925 13925 Misses 1297 1297 Partials 865 865 ```

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