Fix bugs in setting GxSpV_u and GxSpV_v in MOM_isopycnal_slopes

[Wendazhang33]

• In the previous version, GxSpV_u and GxSpV_v were only set when use_EOS was true
• Now initialize GxSpV_u and GxSpV_v to be G_Rho0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 35.87%. Comparing base (90749f3) to head (828e1a0).
Report is 2 commits behind head on dev/gfdl.

Additional details and impacted files

@@             Coverage Diff              @@
##           dev/gfdl     #701      +/-   ##
============================================
- Coverage     36.99%   35.87%   -1.13%     
============================================
  Files           272      269       -3     
  Lines         82438    81375    -1063     
  Branches      15421    15406      -15     
============================================
- Hits          30500    29195    -1305     
- Misses        46246    46438     +192     
- Partials       5692     5742      +50     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Hallberg-NOAA]

I agree that these changes are correct and necessary.

[Hallberg-NOAA]

This PR has passed pipeline testing at https://gitlab.gfdl.noaa.gov/ogrp/mom6ci/MOM6/-/pipelines/24333.