Fix ticker 24hr struct for binance response

[San-B-09]

Fix bug ticker 24hr response struct: ISSUE-52

When calling NewTicker24hrService().Do(ctx) to fetch all symbol info in order to bypass any possible limits I get the following error:
json: cannot unmarshal number -1 into Go struct field Ticker24hrResponse.firstId of type uint64. I suspect this comes from trying to process a symbol which has no trades yet - and the firstId being uninitiliased.