JavaScript extensions get the string "RESERVED" instead of null or undefined when there is no page, context or browser #5320

Describe the bug
A JavaScript extension function that requests page, context or browser receives the string "RESERVED" when that value does not exist. For example: no page is open, or browser is requested while a persistent context is active, where the browser is null.

Python sends ("page", "RESERVED") as a placeholder for every injected argument, and extensionKeywordCall in node/playwright-wrapper/playwright-state.ts replaced it like this:

apiArguments.get(argName) || namedArguments[argName]

When the real value is null or undefined, || falls back to the placeholder, so the extension gets "RESERVED". A guard such as if (!page) therefore never worked, because the string is truthy.

Fix
Fixed in #5318: injected values are passed on as they are, with apiArguments.has(argName) ? apiArguments.get(argName) : namedArguments[argName]. The extension now receives null or undefined.

Release notes
This is a behaviour change for JavaScript extensions. An extension that runs without a page, context or browser now gets null or undefined instead of the string "RESERVED". Only code that compared against the undocumented "RESERVED" marker is affected.

Found while reviewing #5318 (adoptContext, #5317), where browser is null for an adopted context.