Minor Code Improvement: Adjust $mailserver_url Condition Order in wp-mail.php

[dilipbheda]

Reordered the condition in the $mailserver_url check to prioritize the empty() check.

Reason for Change:

• Improves readability and logic consistency by checking for an empty value first.
• Prevents unnecessary string comparison if $mailserver_url is empty.

It follows a common best practice of checking for an empty value before doing a specific comparison.

[mukesh27]

Thanks for ticket and PR.

Move to 6.9

[SergeyBiryukov]

In 60122:

Coding Standards: Check for an empty mail server URL first in wp-mail.php.

This follows a common best practice of checking for an empty value before doing a specific comparison.

Follow-up to [39772].

Props dilipbheda, mukesh27.
Fixes #63219.