message: add tests to shared string to cover null annotations
diff: @@ -109,6 +109,29 @@ describe("SharedString", () => {
  }
  });
  + it("can handle null annotations in text", async () => {
  + const text = "hello world";
  + // eslint-disable-next-line no-null/no-null
  + const startingProps = { style: "bold", color: null };
  + sharedString.insertText(0, text, startingProps);
  +
  + for (let i = 0; i < text.length; i++) {
  + assert.strictEqual(
  + sharedString.getPropertiesAtPosition(i).color, undefined, "Null values allowed in properties");
  + }
  +
  + // eslint-disable-next-line no-null/no-null
  + const updatedProps = { style: null };
  + sharedString.annotateRange(6, text.length, updatedProps);
  +
  + for (let i = 6; i < text.length; i++) {
  + assert.strictEqual(
  + sharedString.getPropertiesAtPosition(i).style,
  + undefined,
  + "Null values allowed in properties");
  + }
  + });
  +
  it("can insert marker", () => {
  sharedString.insertText(0, "hello world");
  // Insert a simple marker.
  