How do I make macOS properly blacken my TV screen on standby
The message "HDMI 1 (Clock) No input. Check your cables, etc." is apparently generated by your TV.  The first place I'd look is in the TV's menu system.  See if there's any way to change what is displayed when there is no incoming HDMI signal.

If you can't find anything there, you can look at the Sony website.  Note that there's a firmware update available for you TV as well.  There's a chance you'll be able to control the "no input" message more easily after the update.

https://www.sony.co.uk/electronics/support/televisions-projectors-lcd-tvs-android-/kdl-43w755c#manuals

But, what you really want, I think, is to set up the Mac Mini so that it shows a blank image as its screensaver.