Improve error handling in get_categories().

When passed an invalid `'taxonomy'`, `get_terms()` will return a `WP_Error`
object. This object should not be blindly cast to an array. Instead, an empty
array should be returned, to indicate that no matching terms have been found.

Props virgodesign, sebastian.pisula.
Fixes #36227.

git-svn-id: https://develop.svn.wordpress.org/trunk@36988 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/category/getCategories.php

@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * @group taxonomy
+ * @group category.php
+ */
+class Tests_Category_GetCategories extends WP_UnitTestCase {
+	/**
+	 * @ticket 36227
+	 */
+	public function test_wp_error_should_return_an_empty_array() {
+		$found = get_categories( array( 'taxonomy' => 'foo' ) );
+		$this->assertSame( array(), $found );
+	}
+}