device-network.c: Fix crash in `pappl_dnssd_list()` with Avahi

[zdohnal]

If PAPPL testsuite runs in isolated environment, e.g. in mock, we cannot generate Avahi client, because Avahi is not running. This causes _papplDNSSDInit() to return NULL, which is later sent into avahi_service_browser_new(), which uses assert() for this argument and make the binary crash if the input is NULL.

The crash makes the testsuite fail during the build, which is useful sanity check before the package is built.

[michaelrsweet]

This shouldn't cause a crash because the cupsDNSSD APIs are sane about NULL checking/ignoring. But happy to add this to short-circuit things...