WP_Filesystem: Change WP_Filesystem_FTPext::exists() and WP_Filesystem_ftpsockets::exists() to return true for empty directories.

Both methods are using *nlist() which returns a list of files in a given directory or the file itself for a given file. If the result was an empty list we assumed that the file doesn't exists. This includes also cases where $file is actually an empty directory. To prevent this we now check if $file is a directory before returning the result of an empty list.
Other filesystem methods are using `file_exists()` which already checks whether a file or directory exists.

fixes #30815.
Built from https://develop.svn.wordpress.org/trunk@31815


git-svn-id: http://core.svn.wordpress.org/trunk@31797 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/class-wp-filesystem-ftpsockets.php

@@ -274,6 +274,11 @@ class WP_Filesystem_ftpsockets extends WP_Filesystem_Base {
 	 */
 	public function exists( $file ) {
 		$list = $this->ftp->nlist( $file );
+
+		if ( empty( $list ) && $this->is_dir( $file ) ) {
+			return true; // File is an empty directory.
+		}
+
 		return !empty( $list ); //empty list = no file, so invert.
 		// Return $this->ftp->is_exists($file); has issues with ABOR+426 responses on the ncFTPd server.
 	}