JP-3541: fixed tweakreg step KeyError due to empty gaia catalog

[emolter]

Resolves JP-3541

Closes #8286

This PR addresses a bug that caused tweakreg to fail when the bounding box of the input image was too small to find any sources in the GAIA catalog. This PR ensures that the appropriate warning (Not enough sources (0) in the reference catalog...), which was already implemented, is hit instead, and the step completes successfully.

Regression test run here.

Checklist for maintainers

• 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
• Make sure the JIRA ticket is resolved properly

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.38%. Comparing base (4cc0ac1) to head (cbf3d78).
Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8334      +/-   ##
==========================================
+ Coverage   75.15%   75.38%   +0.22%     
==========================================
  Files         470      474       +4     
  Lines       38604    38939     +335     
==========================================
+ Hits        29014    29353     +339     
+ Misses       9590     9586       -4     

Flag	Coverage Δ		*Carryforward flag
nightly	77.33% <ø> (-0.07%)	⬇️	Carriedforward from 4cc0ac1

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[hbushouse]

Looks good. Just one minor question.

[hbushouse]

Looks good.