[FIX] component: no crash if empty props

The expression parser properly format an empty string into an empty
string. But then, this empty string was injected in the props object of
a component, and the compiled code looked like this:

let props5 = { val: };

So, in this case, we simply put undefined, since there are no props
value.

closes #587

src/component/directive.ts

@@ -211,7 +211,7 @@ QWeb.addDirective({
       } else if (!name.startsWith("t-")) {
         if (name !== "class" && name !== "style") {
           // this is a prop!
-          props[name] = ctx.formatExpression(value);
+          props[name] = ctx.formatExpression(value) || "undefined";
         }
       }
     }

tests/component/__snapshots__/component.test.ts.snap

@@ -1,5 +1,43 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`basic widget properties can handle empty props 1`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"__template__2\\"
+    let utils = this.constructor.utils;
+    let QWeb = this.constructor;
+    let parent = context;
+    let scope = Object.create(context);
+    let h = this.h;
+    let c1 = [], p1 = {key:1};
+    let vn1 = h('div', p1, c1);
+    // Component 'Child'
+    let w2 = '__3__' in parent.__owl__.cmap ? parent.__owl__.children[parent.__owl__.cmap['__3__']] : false;
+    let props2 = {val:undefined};
+    if (w2 && w2.__owl__.currentFiber && !w2.__owl__.vnode) {
+        w2.destroy();
+        w2 = false;
+    }
+    if (w2) {
+        w2.__updateProps(props2, extra.fiber, undefined);
+        let pvnode = w2.__owl__.pvnode;
+        c1.push(pvnode);
+    } else {
+        let componentKey2 = \`Child\`;
+        let W2 = context.constructor.components[componentKey2] || QWeb.components[componentKey2]|| scope['Child'];
+        if (!W2) {throw new Error('Cannot find the definition of component \\"' + componentKey2 + '\\"')}
+        w2 = new W2(parent, props2);
+        parent.__owl__.cmap['__3__'] = w2.__owl__.id;
+        let fiber = w2.__prepare(extra.fiber, undefined, () => { const vnode = fiber.vnode; pvnode.sel = vnode.sel; });
+        let pvnode = h('dummy', {key: '__3__', hook: {remove() {},destroy(vn) {w2.destroy();}}});
+        c1.push(pvnode);
+        w2.__owl__.pvnode = pvnode;
+    }
+    w2.__owl__.parentLastFiberId = extra.fiber.id;
+    return vn1;
+}"
+`;
+
 exports[`basic widget properties reconciliation alg works for t-foreach in t-foreach 1`] = `
 "function anonymous(context, extra
 ) {

tests/component/component.test.ts

@@ -147,6 +147,21 @@ describe("basic widget properties", () => {
     expect(fixture.innerHTML).toBe("<div>1<button>Inc</button></div>");
   });
 
+  test("can handle empty props", async () => {
+    class Child extends Component<any, any> {
+      static template = xml`<span><t t-esc="props.val"/></span>`;
+    }
+    class Parent extends Component<any, any> {
+      static template = xml`<div><Child val=""/></div>`;
+      static components = { Child };
+    }
+
+    const parent = new Parent();
+    await parent.mount(fixture);
+    expect(env.qweb.templates[Parent.template].fn.toString()).toMatchSnapshot();
+    expect(fixture.innerHTML).toBe( "<div><span></span></div>");
+  });
+
   test("cannot be clicked on and updated if not in DOM", async () => {
     class Counter extends Component<any, any> {
       static template = xml`