handle parameters with valid zero value

[arichardet]

After #321, we need to make a change to handle parameters for which zero is a valid value. Zero is a valid value for user.user gid and uid parameters, and user.group gid parameter. A user can be created by providing a uid or allowing it to be set for you in the system call.

The problem now is that if the parameter is not provided in the configuration, the preparer sets its value to zero.