Filesystem API: Define password as null if not set when using SSH2 with public/private keys.

This resolves an `Undefined array key "password"` PHP warning in `WP_Filesystem_SSH2::connect()` when using public/private key authentication, in which case providing a password is optional.

Follow-up to [8865].

Props J-Dill, costdev, ehsanakhgari, dd32.
Fixes #33196.

git-svn-id: https://develop.svn.wordpress.org/trunk@56111 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/class-wp-filesystem-ssh2.php

@@ -103,6 +103,8 @@ public function __construct( $opt = '' ) {
 			// Password can be blank if we are using keys.
 			if ( ! $this->keys ) {
 				$this->errors->add( 'empty_password', __( 'SSH2 password is required' ) );
+			} else {
+				$this->options['password'] = null;
 			}
 		} else {
 			$this->options['password'] = $opt['password'];