There are two types of parameters; Mandatory and Non-mandatory.
For non-mandatory parameters, empty (" "/$NULL$) data is accepted as data. The action on receiving empty data, accordingly uses the default data for the parameter, however, for the string parameter, it will continue using the empty (zero length string).
For Mandatory parameters, empty (" "/$NULL$) data is not accepted as data and you will need to enter valid data. Custom Actions can have parameters set to mandatory/non-mandatory based on the requirement.