The {{HOSTNAME_OR_URL}} placeholder used for the custom Email notify subject is NULL for Monitor Type=Group

[computrav]

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Description

I customized my email subject to the following:
[{{STATUS}}] {{NAME}} - {{HOSTNAME_OR_URL}}

However, because "Group" monitor types don't have a hostname or URL, the value is Null. That's fine of course, but it appears literally as "null" when used in the email subject, e.g.:
[✅ Up] Main Web Server - null

So the bug is that the {{HOSTNAME_OR_URL}} should be handled properly for monitor types that don't have a hostname/URL (Group and maybe others). Also, perhaps instead of just using an empty string, use "n/a"? For my template above, for example, if it were blank it would show up as:
[✅ Up] Main Web Server -

but instead maybe this is better?
[✅ Up] Main Web Server - n/a

I'm not sure :)

👟 Reproduction steps

1. Setup an email notification and customize the email subject to utilize {{HOSTNAME_OR_URL}}
2. Create a Group monitor and turn on the Email notification for it
3. Add another regular monitor to it
4. Trigger an email (make the monitor UP/DOWN)

👀 Expected behavior

null does not show in the email subject.

😓 Actual Behavior

The literal string "null" shows.

🐻 Uptime-Kuma Version

1.23.16

💻 Operating System and Arch

Debian

🌐 Browser

n/a

🖥️ Deployment Environment

n/a

📝 Relevant log output