<aura:event type="COMPONENT" description="UpdateRPGFlowGraphEvent">
    <aura:attribute name="flow" type="zced__.RPG_Flow__c"/>
    <aura:attribute name="flowId" type="String"/>
</aura:event>

handleButtonClick : function(component, event, helper) {
    let flowVar = component.get("v.flow");
    let flowIdVar = component.get("v.flowId");

    if (flowVar || flowIdVar) {
       //if neither attribute is set, will be skipped
       //but if at least one is set, you're good to go
    } else {
        let toast = $A.get("e.force:showToast");
        // do something that tells the user to try again.
        toast.setParams({
            title: "Oops!",
            message: "You must enter either a flow or flow id value"
       });
       toast.fire();
    }
}