Print n_constrained_dofs when it's non-zero
Print n_constrained_dofs when it's non-zero

This doesn't get triggered often, only when Moose is starting from an
already-adaptively-refined or an IGA-constrained mesh or is enforcing
periodic BCs, but when it's triggered it's useful information.

Refs #20909

bwspenc/moose,sapitts/moose,sapitts/moose,jessecarterMOOSE/moose,milljm/moose,idaholab/moose,dschwen/moose,jessecarterMOOSE/moose,bwspenc/moose,lindsayad/moose,dschwen/moose,harterj/moose,harterj/moose,idaholab/moose,lindsayad/moose,andrsd/moose,laagesen/moose,andrsd/moose,laagesen/moose,idaholab/moose,milljm/moose,harterj/moose,sapitts/moose,andrsd/moose,jessecarterMOOSE/moose,bwspenc/moose,milljm/moose,idaholab/moose,andrsd/moose,bwspenc/moose,lindsayad/moose,milljm/moose,dschwen/moose,bwspenc/moose,harterj/moose,jessecarterMOOSE/moose,laagesen/moose,laagesen/moose,sapitts/moose,jessecarterMOOSE/moose,harterj/moose,dschwen/moose,milljm/moose,lindsayad/moose,laagesen/moose,lindsayad/moose,andrsd/moose,idaholab/moose,dschwen/moose,sapitts/moose