[IMP] t-on directive: allow empty handler

Closes #377

tests/qweb/__snapshots__/qweb.test.ts.snap

@@ -1789,6 +1789,22 @@ exports[`t-on t-on combined with t-raw 1`] = `
 }"
 `;
 
+exports[`t-on t-on with empty handler (only modifiers) 1`] = `
+"function anonymous(context,extra
+) {
+    let owner = context;
+    var h = this.h;
+    let c1 = [], p1 = {key:1};
+    var vn1 = h('div', p1, c1);
+    let c2 = [], p2 = {key:2,on:{}};
+    var vn2 = h('button', p2, c2);
+    c1.push(vn2);
+    p2.on['click'] = function (e) {e.preventDefault();};
+    c2.push({text: \`Button\`});
+    return vn1;
+}"
+`;
+
 exports[`t-on t-on with inline statement (function call) 1`] = `
 "function anonymous(context,extra
 ) {

tests/qweb/qweb.test.ts

@@ -1213,6 +1213,24 @@ describe("t-on", () => {
     expect(steps).toEqual([1, 2]);
   });
 
+  test("t-on with empty handler (only modifiers)", () => {
+    expect.assertions(2);
+    qweb.addTemplate(
+      "test",
+      `<div>
+        <button t-on-click.prevent="">Button</button>
+      </div>`
+    );
+    const node = renderToDOM(qweb, "test", {}, { handlers: [] });
+
+    node.addEventListener('click', (e) => {
+      expect(e.defaultPrevented).toBe(true);
+    });
+
+    const button = (<HTMLElement>node).getElementsByTagName("button")[0];
+    button.click();
+  });
+
   test("t-on combined with t-esc", async () => {
     expect.assertions(3);
     qweb.addTemplate("test", `<div><button t-on-click="onClick" t-esc="text"/></div>`);