Peg related RPC calls should error if there is no parent chain set

Currently (tested in 0.18.1.9 and 0.18.11):

If I set `chain=whatever`, `con_has_parent_chain=0` or `parentgenesisblockhash=Null` in elements.conf I can still call getpeginaddress from a connected client without error and get a `mainchain_address` and `claim_script` returned.

I would have expected any peg related RPC calls to error if the parent chain is nothing.