Fix Gnus duplicate suppression guards (bug#34987)

* lisp/gnus/gnus-dup.el (gnus-dup-enter-articles)
(gnus-dup-suppress-articles): Use gnus-dup-hashtb as an indicator of
initialization instead of gnus-dup-list, which may happen to be nil.
(gnus-dup-unsuppress-article): Do nothing if gnus-dup-hashtb is
uninitialized.
1 job for master in 50 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #1292
test-all

00:50:53

 
Name Stage Failure
failed
test-all Test
Makefile:313: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:288: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:962: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1