udevd: fix a reversed conditional on global property set

  # udevadm control --property=HELLO=WORLD
  Received udev control message (ENV), unsetting 'HELLO'
  # udevadm control --property=HELLO=
  Received udev control message (ENV), setting 'HELLO='

Oh no, it's busted. Let's try removing this one little negation real quick
to see if it helps...

  # udevadm control --property=HELLO=WORLD
  Received udev control message (ENV), setting 'HELLO=WORLD'
  # udevadm control --property=HELLO=
  Received udev control message (ENV), unsetting 'HELLO'

Feels much better now.

(cherry picked from commit f053fc33)