[bug] node/rpc: `DefaultConfig()` for rpc is never invoked for users running older version of node

[renaynay]

From @tzdybal:

I found one issue - default config is not used. and you need to explicitly set --rcp.port to be able to use RPC endpoints. After debugging, found that config is empty, and DefaultConfig is never invoked. After removing existing config and running init, finally default RPC port is set. But this means you need to manually merge configurations on update, etc

Ref: https://github.com/celestiaorg/celestia-node/pull/696

[Bidon15]

Ref: https://github.com/celestiaorg/celestia-node/issues/895

[renaynay]

Resolved by #997 Validate functionality on the config(s)