[FIX] qweb: fix crash when component only renders empty slot

tests/components/__snapshots__/slots.test.ts.snap

@@ -43,6 +43,18 @@ exports[`slots can define and call slots 2`] = `
 }"
 `;
 
+exports[`slots can render only empty slot 1`] = `
+"function anonymous(bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, component } = bdom;
+  let { withDefault, getTemplate, prepareList, withKey, zero, call, callSlot, capture, isBoundary, shallowEqual, setContextValue, toNumber } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    return callSlot(ctx, node, key, 'default');
+  }
+}"
+`;
+
 exports[`slots content is the default slot (variation) 1`] = `
 "function anonymous(bdom, helpers
 ) {

tests/components/slots.test.ts

@@ -1332,4 +1332,19 @@ describe("slots", () => {
 
     expect(fixture.innerHTML).toBe("<div><div><p>Ablip</p><div><p>Bblip</p></div></div></div>");
   });
+
+  test("can render only empty slot", async () => {
+    class Parent extends Component {
+      static template = xml`<t t-slot="default"/>`;
+    }
+
+    let error = null;
+    try {
+      await mount(Parent, fixture);
+    } catch (e) {
+      error = e;
+    }
+    expect(error).toBeNull();
+    expect(fixture.innerHTML).toEqual("");
+  });
 });