app: fix error messages for not specified parameters in argparse

Argparse module stores unspecified parameter values as boolean
true. It led to a problem, that a command line '--value' with
'value' defined as a number or a string, showed a strange error
message:

    Expected number/string, got "true"

Even though a user didn't pass any value. Now it shows 'nothing'
instead of '"true"'. That is clearer.

Follow up #4076