fix #4755
Retention policy is used in Ledger GC and Topic GC.
Default retention policy is not uploaded to zookeeper, but it is getted from zookeeper when it is used.
So, if zookeeper doesn't have retention policy, we should load it from default config file.
In Ledger GC configuration, it's OK.
https://github.com/apache/pulsar/blob/075f28b71c8fd9259ce8e136dbb81c0629c3f271/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/BrokerService.java#L739-L742
But in Topic GC, do nothing.
(cherry picked from commit 1dcac8cc)