When an On Click event in datagrid2 calls a microflow with an empty parameter, the microflow is not called (without error). Only microflow calls with non-empty parameters are called from events. Defining the parameter as optional does change this behavior in some scenarios. When all parameters are defined optional, the microflow is not called. When one parameter is required and non-empty and the rest optional, then the microflow is called.
This behavior is defined by Support as expected, but not (clearly) documented.
However this behavior is not consistent and unclear. A better solution is always call the microflow, independent of the value of the parameters.
The use case for this situation is complex selection functionality in a page that is used for different associations. All endpoints of the possible associations are passed as parameters to the selection page of which only one is non-empty (the one for which the association must be filled). The On Click microflow gets also all these parameters and will fill in the association of the non-empty parameter.