[FIX] qweb: properly display falsy values

While it is not tested, nor documented, the reference QWeb
implementation display the `false` value as "false". So, we have to
adapt the Owl implementation to match that behaviour.

At the same time, this commit uses 'let' instead of 'var' in various places,
to make the compiled code more consistant.

tests/router/__snapshots__/link.test.ts.snap

@@ -6,11 +6,11 @@ exports[`Link component can render simple cases 1`] = `
     // Template name: \\"__template__1\\"
     let utils = this.constructor.utils;
     let scope = Object.create(context);
-    var h = this.h;
+    let h = this.h;
     let _4 = utils.toObj({'router-link-active':scope['isActive']});
-    var _5 = scope['href'];
+    let _5 = scope['href'];
     let c6 = [], p6 = {key:6,attrs:{href: _5},class:_4,on:{}};
-    var vn6 = h('a', p6, c6);
+    let vn6 = h('a', p6, c6);
     extra.handlers['click__7__'] = extra.handlers['click__7__'] || function (e) {if (!context.__owl__.isMounted){return}utils.getComponent(context)['navigate'](e);};
     p6.on['click'] = extra.handlers['click__7__'];
     const slot8 = this.constructor.slots[context.__owl__.slotId + '_' + 'default'];

tests/router/__snapshots__/route_component.test.ts.snap

@@ -9,7 +9,7 @@ exports[`RouteComponent can render simple cases 1`] = `
     let parent = context;
     let scope = Object.create(context);
     let result;
-    var h = this.h;
+    let h = this.h;
     if (scope['routeComponent']) {
         const nodeKey4 = scope['env'].router.currentRouteName;
         // Component 'routeComponent'