[FIX] compiler: dynamic value on inputs doesn't turn 0 into empty string

In #1161, we fixed blockdom treating 0 as falsy when patching a value,
which would result in an empty attribute. It turns out that there is
another place where we had the same fallback, which is when we compute a
dynamic attribute value, so while blockdom had the ability to set the
value to 0, when using a dynamic attribute value we would never give it
0 as a value. This commit changes the fallback strategy for dynamic
attribute values to be the same as the one in blockdom.

closes #1358

tests/compiler/attributes.test.ts

@@ -334,6 +334,14 @@ describe("attributes", () => {
     expect(fixture.innerHTML).toBe('<div value=""></div>');
   });
 
+  test("dynamic input value: falsy values", () => {
+    // 0 doesn't fall back to empty string
+    expect(renderToBdom(`<input t-att-value="0"/>`)).toEqual({ data: [new String("0")] });
+    expect(renderToBdom(`<input t-att-value="false"/>`)).toEqual({ data: [new String("")] });
+    expect(renderToBdom(`<input t-att-value="undefined"/>`)).toEqual({ data: [new String("")] });
+    expect(renderToBdom(`<input t-att-value="''"/>`)).toEqual({ data: [new String("")] });
+  });
+
   test("updating property with falsy value", async () => {
     // render input with initial value
     const template = `<input t-att-value="v"></input>`;