How to write a problemMatcher with "background" and no problems?

[roblourens]

Trying to solve #82621 by adding a problemMatcher with "background" property. But I don't actually want it to match any problems.

If I don't set the "pattern" property I just get

Error: the description can't be converted into a problem matcher:
{
    "background": {
        "activeOnStart": false,
        "beginsPattern": "Web UI available at",
        "endsPattern": "sdf"
    }
}

which is not a helpful error message. But it seems like I have to add an empty problemMatcher pattern like this

"pattern": {
	"regexp": ""
},

[roblourens]

Also, I have to full in a dummy endsPattern, when there is no end output from the task, how can I get rid of that?

[alexr00]

Error messages are unhelpful is a duplicate of #29951

Having a better solution than a problem matcher for tracking background tasks is a duplicate of #6209