Posts, Post Types: Check if the post type exists in wp_insert_post().

This avoids an `Attempt to read property "cap" on null` PHP warning when checking an unregistered post type's `publish_posts` capability to disallow contributors setting the post slug for pending posts.

Follow-up to [9055], [42380].

Props Chouby, mukesh27, rafiahmedd, SergeyBiryukov.
Fixes #55877.

git-svn-id: https://develop.svn.wordpress.org/trunk@53771 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/post.php

@@ -501,6 +501,23 @@ class Tests_Post extends WP_UnitTestCase {
 		$this->assertStringContainsString( 'wptests_pt=' . $p, $post->guid );
 	}
 
+	/**
+	 * @ticket 55877
+	 * @covers ::wp_insert_post
+	 */
+	public function test_wp_insert_post_should_not_trigger_warning_for_pending_posts_with_unknown_cpt() {
+		$post_id = wp_insert_post(
+			array(
+				'post_title'  => 'title',
+				'post_type'   => 'unknown',
+				'post_status' => 'pending',
+			)
+		);
+
+		$this->assertIsNumeric( $post_id );
+		$this->assertGreaterThan( 0, $post_id );
+	}
+
 	/**
 	 * @ticket 20451
 	 */