Remove support for null in patterns

Remove support for specifying the "null" value in "enabled"/"waived" patterns. That is, remove support for matching the "ultimate all members" of a target set.

That was used for specifying the "baseline" runs, which meant that all possible sources were specified at once. That has recently changed to specifying "unknown" sources instead, and sources were removed from targets and patterns altogether, so this is no longer necessary.