`ConstraintsCollection` always added, even when empty

[mattwthompson]

Description

The creation pathway from SMIRNOFF sources results in a ConstraintsCollection always being added, even if there's no corresponding ConstraintHandler or possible way for constraints to be added later.

This is harmless, I think, but also an unexpected result. We could take care to remove it if empty, or simply not create it if no handler is present.

Reproduction

In [1]: from openff.toolkit import Molecule, ForceField

In [2]: sage = ForceField("openff-2.3.0.offxml")

In [3]: sage.deregister_parameter_handler("Bonds")

In [4]: sage.deregister_parameter_handler("Constraints")

In [5]: interchange = sage.create_interchange(Molecule.from_smiles("CCO").to_topology())

In [6]: interchange.collections
Out[6]:
{'Constraints': Handler 'Constraints' with expression '', 0 mapping keys, and 0 potentials,
 'Angles': Handler 'Angles' with expression 'k/2*(theta-angle)**2', 13 mapping keys, and 3 potentials,
 'ProperTorsions': Handler 'ProperTorsions' with expression 'k*(1+cos(periodicity*theta-phase))', 16 mapping keys, and 6 potentials,
 'ImproperTorsions': Handler 'ImproperTorsions' with expression 'k*(1+cos(periodicity*theta-phase))', 0 mapping keys, and 0 potentials,
 'vdW': Handler 'vdW' with expression '4*epsilon*((sigma/r)**12-(sigma/r)**6)', 9 mapping keys, and 5 potentials,
 'Electrostatics': Handler 'Electrostatics' with expression 'coul', 9 mapping keys, and 9 potentials}

A sillier example involves a force field with no handlers at all, which still results in an empty collection being present:

In [8]: [*ForceField().create_interchange(Molecule.from_smiles("CCO").to_topology()).collections.values()]
Out[8]: [Handler 'Constraints' with expression '', 0 mapping keys, and 0 potentials]

Software versions

• Which operating system and version are you using?
• How did you install Interchange?
• What is the output of running conda list?