Request parent.conf via api, if there is no assigned ds's, need to error out clearly.

[jdawgitsome]

When requesting parent.conf via api, and there is no assigned ds's, traffic ops needs to handle and error out clearly.

Currently the error that is reported back is the following:

Traffic Ops fatal error occurred while processing your request.
Error at line 2468 ( foreach my $ds ( sort @{ $data->{dslist} } ) {)
Can't use an undefined value as an ARRAY reference at /opt/traffic_ops/app/lib/API/Configs/ApacheTrafficServer.pm line 2468.

[dg4prez]

#1898