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/Launcher.js

@@ -258,7 +258,7 @@ class Launcher {
   }
 
   /**
-   * @param {!(BrowserOptions & {browserWSEndpoint: string})=} options
+   * @param {!(BrowserOptions & {browserWSEndpoint: string})} options
    * @return {!Promise<!Browser>}
    */
   static async connect(options) {