fix: Prevent undefined get_current_screen function errors (#39228)

* fix: Prevent undefined `get_current_screen` function errors

This function is undefined in certain contexts -- e.g., a REST API
request that results in enqueueing assets. Referencing an undefined
function results in a fatal error. With the `get_current_screen`
function specifically, it is common practice to check the function's
existence prior to invoking the it.

https://developer.wordpress.org/reference/functions/get_current_screen/#comment-5424

* docs: Add change log entry

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/10815999818

Upstream-Ref: Automattic/jetpack@5ee714f

vendor/automattic/jetpack-mu-wpcom/CHANGELOG.md

@@ -12,6 +12,7 @@ This is an alpha version! The changes listed here are not final.
 ### Fixed
 - Admin bar: fix paddings around wpcom and reader logos
 - Launchpad first_post_published task reuses existing draft if there is one
+- Prevent undefined `get_current_screen` function errors resulting from invoking the function in contexts where it is undefined.
 
 ## [5.62.0] - 2024-09-10
 ### Changed

vendor/automattic/jetpack-mu-wpcom/src/features/starter-page-templates/class-starter-page-templates.php

@@ -176,6 +176,10 @@ public function pass_error_to_frontend( $message ) {
 	 * Enqueue block editor assets.
 	 */
 	public function enqueue_assets() {
+		if ( ! function_exists( 'get_current_screen' ) ) {
+			return;
+		}
+
 		$screen      = get_current_screen();
 		$user_locale = Common\get_iso_639_locale( get_user_locale() );