fix: fix null-type bugs (#3137)

I ran TypeScript against our code with `strictNullChecks` on. Most of the errors generated are noise, because TypeScript doesn't understand how our `assert` method works. But some were legitimate bugs. They are fixed in this patch.

lib/Input.js

@@ -38,7 +38,7 @@ class Keyboard {
 
   /**
    * @param {string} key
-   * @param {{text: string}=} options
+   * @param {{text?: string}=} options
    */
   async down(key, options = { text: undefined }) {
     const description = this._keyDescriptionForString(key);