additionalVolumes setting should allow empty values

[pgier]

If I add additionalVolumes: to a resource, but don't add an volumes, I get this error:

PulsarCluster.kaap.oss.datastax.com "pulsar" is invalid: spec.proxy.additionalVolumes: Invalid value: "null": spec.proxy.additionalVolumes in body must be of type object: "null"

This field should allow empty/null values, and similarly the sub-fields volumes and volumeMounts should allow empty config.