Unclear message when DNS resolution is failing during a test on LDAP source

[nqb]

Describe the bug
If you test an AD source and DNS resolution doesn't work inside pfperl-api container, you got a message like this:

Can't connect to server or bind with 'administrator@example.lan' on :389

Related code:

packetfence/lib/pf/Authentication/Source/LDAPSource.pm

Line 610 in e529279

return ($FALSE, "Can't connect to server or bind with '$binddn' on $LDAPServer:$LDAPServerPort");

To Reproduce
Steps to reproduce the behavior:

1. Configure an AD source with host equals to a FQDN (not an IP) which is not resolvable by pfperl-api container
2. Click on Test button inside AD source

=> You received a message from API but you don't know that DNS resolution failed

Expected behavior
API must return a message to explain that DNS resolution has failed.

Additional context
The API already handles this error because we can see this message in packetfence.log:

Jun  1 06:53:13 pfel8dev pfperl-api-docker-wrapper[14153]: pfperl-api(14) ERROR: [mac:[undef]] Unable to resolve ad.example.lan (pf::util::resolve)

I think there is a missing check here to ensure we don't add undefined value in @LDAPServers

packetfence/lib/pf/Authentication/Source/LDAPSource.pm

Lines 240 to 241 in e529279

	# Lookup the server hostnames to IPs so they can be shuffled better and to improve the failure detection
	my @LDAPServers = map { valid_ip($_) ? $_ : @{resolve($_) // []} } @{$self->{'host'} // []};