liqui/yobit: handle empty fetchTrades response
liqui/yobit: handle empty fetchTrades response

- when trade history is empty (but the pair is valid) yobit returns an empty array, rather than a dict from market id to trades.
eg currently for https://yobit.net/api/3/trades/poly_btc (response is `[]`) -- cf dict returned by https://yobit.net/api/3/trades/eth_btc
- note that v3 api supports multiple symbols, eg https://yobit.net/api/3/trades/eth_btc-ltc_btc but we don't use that
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