[FIX] compiler: fix falsy values for properties not keeping input empty

Recently, we made it so that when a component is rendered, it always
updates the property values for computed properties. This was done by
wrapping the value in a String or Boolean object. One issue with this is
that wrapping a falsy value in a String doesn't yield an empty string,
but a string containing the value as text (eg new String(undefined) ->
"undefined"), which causes the value to not remain empty as per the
spec. This commit fixes that by adding a fallback to the empty string
for falsy values before converting to a String object.

closes: #1236

tests/compiler/attributes.test.ts

@@ -329,6 +329,35 @@ describe("attributes", () => {
     expect(fixture.innerHTML).toBe('<div value=""></div>');
   });
 
+  test("updating property with falsy value", async () => {
+    // render input with initial value
+    const template = `<input t-att-value="v"></input>`;
+    const bnode1 = renderToBdom(template, { v: false });
+    const fixture = makeTestFixture();
+    mount(bnode1, fixture);
+
+    const input = fixture.querySelector("input")!;
+    expect(input.value).toBe("");
+
+    patch(bnode1, renderToBdom(template, { v: "owl" }));
+    expect(input.value).toBe("owl");
+
+    patch(bnode1, renderToBdom(template, { v: false }));
+    expect(input.value).toBe("");
+
+    patch(bnode1, renderToBdom(template, { v: "owl" }));
+    expect(input.value).toBe("owl");
+
+    patch(bnode1, renderToBdom(template, { v: undefined }));
+    expect(input.value).toBe("");
+
+    patch(bnode1, renderToBdom(template, { v: "owl" }));
+    expect(input.value).toBe("owl");
+
+    patch(bnode1, renderToBdom(template, { v: null }));
+    expect(input.value).toBe("");
+  });
+
   test("changing a class with t-att-class", () => {
     // render input with initial value
     const template = `<div t-att-class="v"/>`;