Tests: Align null return type for get_registered_src().

Update the test and the caller in load_script_module_textdomain() to
match the ?string return type of WP_Script_Modules::get_registered_src(),
which returns null (not false) for unregistered modules.

Author: manzoorwanijk

src/wp-includes/l10n.php

@@ -1300,7 +1300,7 @@ function load_script_module_textdomain( $id, $domain = 'default', $path = '' ) {
 
 	$src = wp_script_modules()->get_registered_src( $id );
 
-	if ( false === $src ) {
+	if ( null === $src ) {
 		return false;
 	}
 

tests/phpunit/tests/script-modules/wpScriptModules.php

@@ -2618,15 +2618,15 @@ public function test_wp_set_script_module_translations_returns_false_for_unregis
 	}
 
 	/**
-	 * Tests that get_registered_src() returns false for unregistered module.
+	 * Tests that get_registered_src() returns null for unregistered module.
 	 *
 	 * @ticket 65015
 	 *
 	 * @covers WP_Script_Modules::get_registered_src
 	 */
-	public function test_get_registered_src_returns_false_for_unregistered_module() {
+	public function test_get_registered_src_returns_null_for_unregistered_module() {
 		$result = $this->script_modules->get_registered_src( 'unregistered-module' );
-		$this->assertFalse( $result );
+		$this->assertNull( $result );
 	}
 
 	/**