[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/qweb/qweb.test.ts

@@ -152,6 +152,26 @@ describe("t-esc", () => {
     const domRendered = renderToDOM(qweb, "testCaller") as HTMLElement;
     expect(domRendered.querySelector("span")!.textContent).toBe("<p>escaped</p>");
   });
+
+  test("div with falsy values", () => {
+    qweb.addTemplate("test", `
+      <div>
+        <p t-esc="v1"/>
+        <p t-esc="v2"/>
+        <p t-esc="v3"/>
+        <p t-esc="v4"/>
+        <p t-esc="v5"/>
+      </div>`);
+    const vals = {
+      v1: false,
+      v2: undefined,
+      v3: null,
+      v4: 0,
+      v5: ""
+    }
+    expect(renderToString(qweb, "test", vals)).toBe("<div><p>false</p><p></p><p></p><p>0</p><p></p></div>");
+  });
+
 });
 
 describe("t-raw", () => {