Allow CyIpopt to solve problems without objectives

[Robbybp]

Fixes

CyIpopt can't solve problems without objectives because it fails to create the PyomoNLP that it needs to construct the CyIpoptProblemInterface

Summary/Motivation:

Use CyIpopt as a solver for square problems in various contexts.

Changes proposed in this PR:

• In PyomoCyIpoptSolver.solve, check the model for active objectives. If none, add a dummy objective with value zero that is removed after the NLP object is created.

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[Robbybp]

Test failure appears unrelated:

FAILED pyomo/solvers/tests/mip/test_scip_log_data.py::test_scip_some_more - AssertionError: assert 'Gap' in {'Status': 'ok', 'Message': 'time limit reached', 'Termination condition': 'maxTimeLimit', 'Id': 403, 'Error rc': 0, 'Time': 0.025727272033691406}