Hide repeat send option if sending nothing

to address https://discourse.nodered.org/t/trigger-node-how-to-delay-and-repeat-message/74117/5

packages/node_modules/@node-red/nodes/core/function/89-trigger.html

@@ -25,7 +25,7 @@
         <select id="node-then-type" style="width:70%;">
             <option value="block" data-i18n="trigger.wait-reset"></option>
             <option value="wait" data-i18n="trigger.wait-for"></option>
-            <option value="loop" data-i18n="trigger.wait-loop"></option>
+            <option id="node-trigger-wait-loop" value="loop" data-i18n="trigger.wait-loop"></option>
         </select>
     </div>
     <div class="form-row node-type-duration">
@@ -181,6 +181,13 @@
                 $("#node-input-op2type").val('str');
             }
 
+            $("#node-input-op1").on("change", function() {
+                if ($("#node-input-op1type").val() === "nul") {
+                    $("#node-trigger-wait-loop").hide();
+                }
+                else { $("#node-trigger-wait-loop").show(); }
+            });
+
             var optionNothing = {value:"nul",label:this._("trigger.output.nothing"),hasValue:false};
             var optionPayload = {value:"pay",label:this._("trigger.output.existing"),hasValue:false};
             var optionOriginalPayload = {value:"pay",label:this._("trigger.output.original"),hasValue:false};