fix: use is_resource() instead of null check for SMTP connection validation

Author: michalsn

system/Email/Email.php

@@ -2227,7 +2227,7 @@ protected function mimeTypes($ext = '')
 
     public function __destruct()
     {
-        if ($this->SMTPConnect !== null) {
+        if (is_resource($this->SMTPConnect)) {
             try {
                 $this->sendCommand('quit');
             } catch (ErrorException $e) {

user_guide_src/source/changelogs/v4.6.3.rst

@@ -31,6 +31,7 @@ Bugs Fixed
 **********
 
 - **Email:** Fixed a bug with CID check when building email attachments.
+- **Email:** Fixed SMTP connection resource validation in the class destructor.
 
 See the repo's
 `CHANGELOG.md <https://github.com/codeigniter4/CodeIgniter4/blob/develop/CHANGELOG.md>`_