Custom tasks can't be customized

[dbaeumer]

Tests #71089

• contribute a custom task as described in the test plan item
• Execute Terminal > Customize Task
• select the contributed task

Observe: the tasks.json opens but the task is not inserted. You get

{
	// See https://go.microsoft.com/fwlink/?LinkId=733558 
	// for the documentation about the tasks.json format
	"version": "2.0.0",
	"tasks": [
		{
			"problemMatcher": []
		}
	]
}

[alexr00]

You can manually customize them, but it looks like the Customize Task command doesn't work for them. Good find.