Safeguard the APGD solver for the case of zero constraints.
Safeguard the APGD solver for the case of zero constraints.

- early termination is possible right after the call to ShurBvectorCompute where M^-1 * f is distributed to the variables.
- this prevents divisions by zero and potential infinite loops

Milad-Rakhsha/chrono,rserban/chrono,dariomangoni/chrono,rserban/chrono,projectchrono/chrono,Milad-Rakhsha/chrono,rserban/chrono,rserban/chrono,projectchrono/chrono,dariomangoni/chrono,dariomangoni/chrono,projectchrono/chrono,dariomangoni/chrono,Milad-Rakhsha/chrono,Milad-Rakhsha/chrono,projectchrono/chrono,projectchrono/chrono,rserban/chrono,rserban/chrono,Milad-Rakhsha/chrono,rserban/chrono,Milad-Rakhsha/chrono,dariomangoni/chrono,projectchrono/chrono,dariomangoni/chrono