dnssources: ignore empty targets for slave entries on update

[MartinWeindel]

What this PR does / why we need it:
If an ingress controller is updated, its load balancer may be recreated. As a consequence, all related ingress resources lose their .status.loadBalancer values and their DNS entries are deleted temporarily.
To avoid such "hickups", on updating DNS entries for a DNS source object, an empty target list is ignored and the old targets are just kept until new targets (e.g. loadbalancer addresses) are set.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

Ignore empty targets for slave entries on update. More concretely, if an ingress resource lose their load balancer addresses during an update, the old targets are kept until new targets are set.