Guard mailbox fetch against missing rows

[onyangodonomondi]

• I've read the contribution guidelines and wholeheartedly agree them

What does this PR include?

Short Description

This PR adds a guard in data/web/inc/functions.mailbox.inc.php so the mailbox details lookup returns false when no database row is found.

Without this check, the code can continue accessing $row[...] after an empty query result, which risks warnings or follow-on failures for invalid or missing mailbox lookups.

Affected Containers

• php-fpm-mailcow

Did you run tests?

What did you tested?

• Ran php -l inside php-fpm-mailcow for data/web/inc/functions.mailbox.inc.php
• Logged into the local demo environment as demo@demo.localhost
• Called the live mailbox details API for an existing mailbox
• Called the live mailbox details API for a nonexistent mailbox
• Confirmed the local UI still responds with HTTP 200 OK

What were the final results? (Awaited, got)

Awaited:

• Existing mailbox lookups should continue returning normal mailbox details
• Missing mailbox lookups should fail safely without warnings or fatal errors

Got:

• Existing mailbox lookup returned the expected mailbox details for demo@demo.localhost
• Missing mailbox lookup returned {} cleanly for missing-mailbox-check@mail.localhost
• No syntax errors were detected in the updated PHP file
• The local UI continued serving normally