Ensure PyROS Supports Problems Containing No Variables/Uncertain Parameters

[shermanjasonaf]

Summary/Motivation:

As an example from #2555 suggests, PyROS is (now) capable of solving RO models that do not have variables appearing in the model's active objective or constraints. However, PyROS currently raises an exception if the user-supplied arguments first_stage_variables and second_stage_variables are (cast to) empty lists.

Further, PyROS is also capable of solving problems that have no uncertain parameters (efficiently, as of #3503). However, this is not (fully) checked in the testing modules.

Changes proposed in this PR:

• Ensure PyROS supports problems with no (user-supplied first-stage or second-stage) variables appearing in the active objective/constraint expressions
• Add test ensuring that PyROS supports problems with no user-provided uncertain parameters

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[shermanjasonaf]

@jas-yao @natalieisenberg

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (e173cb5) to head (18d56e4).
⚠️ Report is 48 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3720   +/-   ##
=======================================
  Coverage   89.31%   89.31%           
=======================================
  Files         896      896           
  Lines      103682   103684    +2     
=======================================
+ Hits        92605    92607    +2     
  Misses      11077    11077           

Flag	Coverage Δ
builders	29.11% <20.00%> (+0.01%)	⬆️
default	85.93% <100.00%> (?)
expensive	35.86% <20.00%> (?)
linux	86.96% <100.00%> (-2.09%)	⬇️
linux_other	86.96% <100.00%> (+<0.01%)	⬆️
osx	83.10% <100.00%> (+<0.01%)	⬆️
win	85.21% <100.00%> (+<0.01%)	⬆️
win_other	85.21% <100.00%> (+<0.01%)	⬆️

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[jsiirola]

Looks pretty good. One implementation change that would be nice, and maybe a little more information in the warnings?