Update dagintegritytestdefault.py

[sunkickr]

Description

The new test fails when the file ".astro/dag_integrity_exceptions.txt" does not exist because we do not handle that case in the test code. This PR fixes that issue

🎟 Issue(s)

Related #XXX

🧪 Functional Testing

Tested manually

📸 Screenshots

📋 Checklist

• [ ] Rebased from the main (or release if patching) branch (before testing)
• [ ] Ran make test before taking out of draft
• [ ] Ran make lint before taking out of draft
• [ ] Added/updated applicable tests
• [ ] 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 86.65%. Comparing base (d705171) to head (8920cfa).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1661 +/- ## ======================================= Coverage 86.65% 86.65% ======================================= Files 114 114 Lines 16712 16712 ======================================= Hits 14481 14481 Misses 1338 1338 Partials 893 893 ```

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