Skip sending transaction if it includes no prices

[zarazan]

Error output from price-feeder when starting:

7:53PM DBG failed to broadcast tx; retrying... error="rpc error: code = Unknown desc = rpc error: code = Unknown desc = must provide at least one oracle exchange rate: invalid request [cosmossdk.io/errors@v1.0.0-beta.7/errors.go:149] With gas wanted: '0' and gas used: '6666' : unknown request" last_check_height=299825 max_height=299830 module=oracle_client tx_code=0 tx_hash=
Mar 06 19:53:56 devnet-n1 price-feeder[252390]: 7:53PM DBG failed to broadcast tx; retrying... error="rpc error: code = Unknown desc = rpc error: code = Unknown desc = must provide at least one oracle exchange rate: invalid request [cosmossdk.io/errors@v1.0.0-beta.7/errors.go:149] With gas wanted: '0' and gas used: '6666' : unknown request" last_check_height=299826 max_height=299830 module=oracle_client tx_code=0 tx_hash=