Register validator topics

[mosmartin]

Expected behavior

When registering a validator, the node should not err on empty topics as there should be a default one.

Actual behavior

The node logs an error
[moduleName=pos commandName=registerValidator err=Topics must have at least one element.] Command execution failed
when trying to register a validator.

Steps to reproduce

Run the command to register a validator

• ./bin/run transaction create pos registerValidator {FEE}
• Follow the prompts
• ./bin/run transaction send {txn}
• Observe the node logs

Which version(s) does this affect? (Environment, OS, etc...)

• Using the example pos application in the development branch