s3-printing: fix cups pcap reload with no printers

s3-printing: fix cups pcap reload with no printers

cups_async_callback() is called to receive new printcap data from a
child process which requests the information from cupsd.
Newly received printcap information is stored in a temporary printcap
cache (tmp_pcap_cache). Once the child process closes the printcap IPC
file descriptor, the system printcap cache is replaced with the newly
populated tmp_pcap_cache, however this only occurs if tmp_pcap_cache is
non null (has at least one printer).

If the printcap cache is empty, which is the case when cups is not
exporting any printers, the printcap cache is not replaced resulting in
stale data.

https://bugzilla.samba.org/show_bug.cgi?id=7915
Signed-off-by: Andreas Schneider <asn@samba.org>