Fix PHP 5.2 notice by ensuring $memo is always an array (#7120)

* Failing test case for PHP 5.2 throwing notice

* Use a scalar value, not null, to truly reproduce the issue

* Fix PHP 5.2 notice by ensuring `$memo` is always an array

`array_reduce()` only permits `$initial` to be a mixed variable as of PHP 5.3

* Add a comment to why this is here

lib/client-assets.php

@@ -487,6 +487,13 @@ function gutenberg_register_scripts_and_styles() {
  * @return array Modified reduce accumulator.
  */
 function gutenberg_preload_api_request( $memo, $path ) {
+
+ // array_reduce() doesn't support passing an array in PHP 5.2
+ // so we need to make sure we start with one.
+ if ( ! is_array( $memo ) ) {
+ $memo = array();
+ }
+
  if ( empty( $path ) ) {
  return $memo;
  }

phpunit/class-admin-test.php

@@ -151,4 +151,13 @@ function test_gutenberg_revisions_restore() {
  $link = apply_filters( 'wp_prepare_revision_for_js', array( 'restoreUrl' => 'http:https://test.com' ) );
  $this->assertEquals( array( 'restoreUrl' => 'http:https://test.com' ), $link );
  }
+
+ /**
+ * Ensure gutenberg_preload_api_request() works without notices in PHP 5.2.
+ *
+ * The array_reduce() function only accepts mixed variables starting with PHP 5.3.
+ */
+ function test_preload_api_request_no_notices_php_52() {
+ $this->assertTrue( is_array( gutenberg_preload_api_request( 0, '/' ) ) );
+ }
 }