Exit Pipeline if user did not enter values for Active Choice Parameterized pipeline build

I wish to abort the pipeline if the user did not select any value for Active Choice parameter for single/multi choice/string pipeline parameter.

For example I have Active Choices Reactive Parameter Named "IPAddress" of Type "Multi Select" with Groovy Script as below:

if (Location.equals("MyTown")) {
return["DDL1", "DDL2", "DDL3", "DDL4"]
} else if (Location.equals("Your Town")) {
return["DDP1", "DDP2"]
} else {
return ["Select an IP from the drop-down"]
}

Thus, once I run the pipeline i see "Select an IP from the drop-down" for IPAddress.

Now, If the user does not select anything from the dropdown the pipeline should fail & abort.

In the pipeline script I have written the below condition check which fails to check the condition despite user ignoring to select any IPAddress.

def ex(param){
    currentBuild.result = 'ABORTED'
    error('BAD PARAM: ' + param)
}

pipeline {          

    agent any       

        stages {

            stage ("Pre-Check Parameters") {        

                steps {

                echo "Pre-Check called in pipeline"

                 script {
                    if ("${params.IPAddress}" == null) {ex("IPAddress")}
                    //if ("${params.myEnv}" == null) {ex("b")}
                    //if ("${params.myLoc}" == null) {ex("c")}
                    }            
               }              
             } 
          }
      }

Can you please suggest what could be the issue here ?