fixed bug that crashed retraining if no alignment pairs were generated for any users
fixed bug that crashed retraining if no alignment pairs were generated for any users

Musi13/bwi_experimental,utexas-bwi/bwi_experimental,Musi13/bwi_experimental,Musi13/bwi_experimental,utexas-bwi/bwi_experimental,Musi13/bwi_experimental,pato/bwi_experimental,Musi13/bwi_experimental,utexas-bwi/bwi_experimental,utexas-bwi/bwi_experimental,utexas-bwi/bwi_experimental,Musi13/bwi_experimental,pato/bwi_experimental,utexas-bwi/bwi_experimental,pato/bwi_experimental,pato/bwi_experimental,pato/bwi_experimental,pato/bwi_experimental