"Return False" resulting java.lang.NumberFormatException: For input string: "false”

My customer is writing a script to access value of timestamp so that once timestamp attribute does not contain value, it will write data otherwise it will not take any action. Therefore customer used "return false".

Script works fine when attribute is empty - inputs timestamp value, however when the attribute already contain value, it will generate java.lang.NumberFormatException: For input string: "false” in log file.

Could you please refer to below script and advise how script will need to be modified in order to avoid current error?

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dojo.provide("com.ibm.providers.script.TimeStampTest");

(function() {
    dojo.declare("com.ibm.providers.script.TimeStampTest", null, {
   
        getValue: function(attribute, workItem, configuration) {
            dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");
            dojo.require("dojo.date.stamp");
       
            var WorkItemAttributes = com.ibm.team.workitem.api.common.WorkItemAttributes;
            var WorkItemState = workItem.getValue(WorkItemAttributes.STATE);
           
            if ( WokkItemState == "com.ibm.workflow.state.s4" ) {
                 var cm_act_start_date= dojo.date.stamp.fromISOString(workItem.getValue("com.ibm.workitem.attribute.cm_act_start_date"));
                
                 if (cm_act_start_date == null){
                     var currentDate = new Date();
                     var currentDateString= dojo.date.stamp.toISOString(currentDate, {milliseconds:true, zulu:true});
                     
                     return currentDateString;
                 } else {
                     return false;
                 }
             } else {
                return false;
             }
        }
    });
})();
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