statuscake_pagespeed_check make alert_config optional

[floh96]

Is your feature request related to a problem? Please describe.
I have some page speed tests where I don't want any alarms because I only want to get the results via reporting.
It is possible to disable all alarms in terraform, but you must explicitly set all alerts to 0.

Describe the solution you'd like
Make alert_config optional with default values of 0 like in the UI.