NiFi does not start in cluster mode without sensitive key explicitly defined

While the docs instruct users to explicitly set a value for nifi.sensitive.props.key in nifi.properties to use when encrypting sensitive component properties to persist in the flow.xml.gz, some users do not. Historically, these instances use a default, hardcoded key generation value.

During the deprecation of StringEncryptor.createEncryptor(NiFiProperties) in NIFI-5376, the code which provided the default was missed in a specific cluster scenario.

The default should be provided, and a warning should be printed to the app log if the provided key value is empty, in both standalone and cluster modes.