time_skew: Do not print message if trust anchor for . is not configured

Without this check, the module would print warning for every single run
without root trust anchor.
9 jobs for detect-time-skew in 2 minutes and 27 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Build
passed #83748
docker linux amd64
build:linux:amd64

00:00:30

 
  Test
passed #83752
docker linux amd64
deckard:linux:amd64

00:00:34

passed #83751
docker linux amd64
installcheck:linux:amd64

00:00:23

passed #83753
docker linux amd64
installcheck:valgrind:linux:amd64

00:00:29

passed #83749
docker
lint:lua

00:00:18

passed #83755
docker linux amd64
respdiff:linux:amd64

00:01:40

passed #83750
docker linux amd64
test:linux:amd64

00:00:23

passed #83754
docker linux amd64
test:linux:amd64:valgrind

00:00:30

 
  Deploy
passed #83756
docker linux amd64
coverage:linux:amd64

00:00:16

65.4%