Bitfinex - Position `openPrice` it's not always present on parsed positions

[Torres-ssf]

Right the Bitfinex position parse method uses the field trade_amount that is inside the meta prop. Turns out that this prop is not always present in the API response.

  interface IBitfinexPositionMeta {
    reason: 'TRADE' | 'FUNDING_ACCR' 
    order_id: number
    liq_stage: null | number
    order_cid: null | number
    order_gid: null | number
    trade_price?: string
    trade_amount?: string
    order_id_oppo: number
  }