DiscreteVariable.val_from_str_add: Convert passed value to string
DiscreteVariable.val_from_str_add: Convert passed value to string

Corrected method expect strings as passed arguments. In case of reading excel file, passed arguments type could be integer or float as well.
So in the method all not None values are converted to strings making sure the DiscreteVariable is properly generated.

kwikadi/orange3,cheral/orange3,marinkaz/orange3,cheral/orange3,qPCR4vir/orange3,kwikadi/orange3,qPCR4vir/orange3,kwikadi/orange3,kwikadi/orange3,cheral/orange3,qPCR4vir/orange3,marinkaz/orange3,marinkaz/orange3,marinkaz/orange3,marinkaz/orange3,marinkaz/orange3,cheral/orange3,kwikadi/orange3,qPCR4vir/orange3,qPCR4vir/orange3,cheral/orange3,kwikadi/orange3,qPCR4vir/orange3,cheral/orange3