Update to prevent divide by zero in future version of mstar

[wfcooke]

Also standardizing capitalization of names of some variables (MStar vs mstar)

[codecov-io]

Codecov Report

Merging #994 into dev/gfdl will decrease coverage by <.01%.
The diff coverage is 7.69%.

@@             Coverage Diff              @@
##           dev/gfdl     #994      +/-   ##
============================================
- Coverage     43.24%   43.24%   -0.01%     
============================================
  Files           213      213              
  Lines         62214    62216       +2     
============================================
  Hits          26905    26905              
- Misses        35309    35311       +2

Impacted Files	Coverage Δ
...c/parameterizations/vertical/MOM_energetic_PBL.F90	66.46% <7.69%> (-0.17%)	⬇️

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[Hallberg-NOAA]

I agree with the logic with this change. It remains a mystery to me why the product of MStar and UStar should be zero in an ocean cell, although this was obviously what was happening at the beginning of the SPEAR test cases where @wfcooke was encountering the problems.

I also appreciate the changes for more consistent capitalization of variables. Assuming that this passes the pipeline tests, this PR should be accepted.

[Hallberg-NOAA]

The pipeline testing this PR has passed at https://gitlab.gfdl.noaa.gov/ogrp/MOM6/pipelines/8860.