Fix failure in niriss tweakreg test

[mcara]

This PR addresses an issue where a regression test may fail even though the code is working as expected (I think).

Checklist for PR authors (skip items if you don't have permissions or they are not applicable)

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• All comments are resolved
• Make sure the JIRA ticket is resolved properly

[mcara]

https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1452/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.01%. Comparing base (781e0e0) to head (a0c3f14).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8489      +/-   ##
==========================================
+ Coverage   57.93%   58.01%   +0.07%     
==========================================
  Files         387      387              
  Lines       38839    38811      -28     
==========================================
+ Hits        22502    22515      +13     
+ Misses      16337    16296      -41     

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[hbushouse]

Another regtest run started at https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1456

[mcara]

Another try: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1457/

[hbushouse]

Looks good now. All regtests are passing, including this new one to test the failure condition (step skipped when no sources are found).