fix incorrect boolean comparison within isfile()

[zacharyburnett]

This PR addresses problems brought by sonarqube: https://plsonarqube.stsci.edu/project/issues?resolved=false&inNewCodePeriod=true&types=BUG&id=romancal&open=AY8c5NNYlU1kwdd-S5_H

Checklist

• added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.05%. Comparing base (b61e70b) to head (69485b3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1210   +/-   ##
=======================================
  Coverage   79.05%   79.05%           
=======================================
  Files         116      116           
  Lines        7994     7994           
=======================================
  Hits         6320     6320           
  Misses       1674     1674           

Flag	Coverage Δ		*Carryforward flag
nightly	62.78% <ø> (ø)		Carriedforward from d00e8d9

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[zacharyburnett]

regtests running at https://plwishmaster.stsci.edu:8081/blue/organizations/jenkins/RT%2FRoman-Developers-Pull-Requests/detail/Roman-Developers-Pull-Requests/723/pipeline

[schlafly]

LGTM, thanks!