Allow having mortar meshes without corresponding mortar constraints

[lindsayad]

Reason

With the creation of mortar aux kernels, it is theoretically possible, for example, that a user wants to compute an aux quantity on a mortar interface in the reference configuration whereas they want the mortar constraint to execute in the displaced configuration. Currently our logic in the constraint warehouse and NonlinearSystemBase doesn't support this use.

Design

Create hasActiveMortarConstraints method in the ConstraintWarehouse and then some small other changes.

Impact

Allow seamless combinations of reference and displaced mesh mortar computations.