JP-3234: Mask NaN values for background subtraction of "robust mean"

[tapastro]

Resolves JP-3234

Closes #

This PR addresses a bug in the calculation of the mean background value when NaN values are present.

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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f3a6047) 77.46% compared to head (2a836ec) 77.47%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7587   +/-   ##
=======================================
  Coverage   77.46%   77.47%           
=======================================
  Files         456      456           
  Lines       36536    36538    +2     
=======================================
+ Hits        28304    28306    +2     
  Misses       8232     8232           

Flag	Coverage Δ		*Carryforward flag
nightly	77.55% <ø> (-0.01%)	⬇️	Carriedforward from f3a6047

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
jwst/background/background_sub.py	93.57% <100.00%> (+0.12%)	⬆️

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[hbushouse]

Looks good. Have you started a regtest run?

[tapastro]

Whoops, started one here: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/722/

[tapastro]

Regtests show only expected errors - ifu cubepars issues and a reference file update for LRS fixed slit extract1d parameters.