Prevent PHP error on invalid excluded extensions check (#45046)

Author: tbradsha

projects/plugins/jetpack/changelog/fix-jetpack-fatal_on_invalid_excluded_extensions

@@ -0,0 +1,4 @@
+Significance: patch
+Type: other
+
+Prevent PHP error when checking for available Gutenberg extensions.

projects/plugins/jetpack/class.jetpack-gutenberg.php

@@ -328,6 +328,11 @@ public static function get_available_extensions( $allowed_extensions = null ) {
 		$exclusions         = get_option( 'jetpack_excluded_extensions', array() );
 		$allowed_extensions = $allowed_extensions === null ? self::get_jetpack_gutenberg_extensions_allowed_list() : $allowed_extensions;
 
+		// Avoid errors if option data is not as expected.
+		if ( ! is_array( $exclusions ) ) {
+			$exclusions = array();
+		}
+
 		return array_diff( $allowed_extensions, $exclusions );
 	}