Fixing spurious DoF in Gibbs reactor model

[andrewlee94]

Fixes #1392

Summary/Motivation:

The current implementation of the Gibbs reactor model includes all elements in the Lagrange multipliers expressions, even if the elemental composition for a given component is 0. This leads to spurious degrees of freedom where the variable appears to be in active constraints but is multiplied y zero (and thus filtered out in the Pyomo solver writers).

Changes proposed in this PR:

• Add logic to catch these cases and exclude them from the expression.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.62%. Comparing base (f8ccdec) to head (59dc412).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1393 +/- ## ========================================== - Coverage 77.63% 77.62% -0.01% ========================================== Files 391 391 Lines 64375 64386 +11 Branches 14257 14262 +5 ========================================== + Hits 49978 49981 +3 - Misses 11827 11834 +7 - Partials 2570 2571 +1 ```

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