[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";
         }
       }
     }