[IMP] qweb: turn handlers into function expressions only

For the sake of consistency with vanilla JS, and to allow some things
that were previously not possible.

src/component/component_node.ts

@@ -93,7 +93,7 @@ export class ComponentNode<T extends typeof Component = any> implements VNode<Co
     this.level = parent ? parent.level + 1 : 0;
     applyDefaultProps(props, C);
     this.component = new C(props, app.env, this) as any;
-    this.renderFn = app.getTemplate(C.template).bind(null, this.component, this);
+    this.renderFn = app.getTemplate(C.template).bind(this.component, this.component, this);
     if (C.style) {
       applyStyles(C);
     }

src/component/handler.ts

@@ -26,12 +26,11 @@ export const mainEventHandler = (data: any, ev: Event, currentTarget?: EventTarg
       }
     }
   }
-  if (typeof data[0] === "function") {
-    data[0](ev);
-  } else if (data[0].__owl__) {
-    const method = data[1];
-    const args = data[2] || [];
-    data[0].__owl__.component[method](...args, ev);
+  // If handler is empty, the array slot 0 will also be empty, and data will not have the property 0
+  // We check this rather than data[0] being truthy (or typeof function) so that it crashes
+  // as expected when there is a handler expression that evaluates to a falsy value
+  if (Object.hasOwnProperty.call(data, 0)) {
+    data[0].call(data[1] ? data[1].__owl__.component : null, ev);
   }
   return stopped;
 };