Fixes # .

Summary/Motivation:

NOTE: This is built on top of #3651, so it will be impossible to review until that is merged.

This fixes a couple bugs in the core.lp_dual transformation. In particular:

1. The Parameterized Standard Form walker now checks if a Var is being treated as a parameter before checking if it is fixed--this is very important because it mean that we were losing fixed Vars we were treating as data, leading to unexpected behavior in dualize-and-combine methods.
2. We now raise an error for completely empty primal models because we don't actually do the right thing in the sense that strong duality won't hold (whatever that means), so I'd rather just not do anything
3. We now raise an error when parameterization causes a primal Constraint to become trivial because, unlike with trivial constraints not due to parameterization, we cannot just ignore them, but the behavior from taking the dual with them in
   (whatever that means) is rather unexpected, I think.
4. We now raise an error when the primal model has discrete variables (or other domains we can't handle).

Changes proposed in this PR:

• Fixing the three bugs above, the first of which is actually a change to parameterized standard form
• Adding tests

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