new-muc-dialog: Don't warn non existing accounts

purple can have accounts with unsupported protocols.  Chatty
shall not have an object associated with it.  Don't warn in that cases
7 jobs for wip/sadiq/no-warn-account in 10 minutes and 26 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #367041
librem5
build-pureos-gcc-byzantium

00:02:09

 
  Test
passed #367043
librem5
check-po

00:00:16

passed #367042
librem5
test:debian-gcc

00:03:15

23.0%
 
  Package
passed #367044
librem5
package-pureos-byzantium

00:01:35

passed #367045
librem5:arm64
package-pureos-byzantium:arm64

00:04:59

 
  Test Package
passed #367046
librem5
autopkgtest-pureos-byzantium-package

00:01:14

passed #367047
librem5
lintian-pureos-byzantium-package

00:00:43