Some trading pairs cannot be decoded

[miohtama]

Fixing pair decoding if there are NaN values from the server.

DataDecodeFailed: Could not decode trading pair data:
 {'address': '0x0d4a11d5eeaac28ec3f61d100daf4d40471f1852',
 'base_token_symbol': 'WETH',
 'buy_count_30d': 26040.0,
 'buy_count_all_time': 2556015.0,
 'buy_tax': 0.0,
 'buy_volume_30d': 41786724.0,
 'buy_volume_all_time': 19510417408.0,
 'chain_id': 1,
 'dex_type': 'uniswap_v2',
 'exchange_address': '0x5c69bee701ef814a2b6a3edd4b1652cb9cc5aa6f',
 'exchange_id': 1,
 'exchange_slug': 'uniswap-v2',
 'fee': 30,
 'first_swap_at': nan,
 'first_swap_at_block_number': nan,
 'flag_blacklisted_manually': None,
 'flag_inactive': None,
 'flag_unknown_exchange': None,
 'flag_unsupported_quote_token': None,
 'last_swap_at': nan,
 'last_swap_at_block_number': nan,
 'pair_id': 239,
 'pair_slug': 'eth-usdt',
 'quote_token_symbol': 'USDT',
 'sell_count_30d': 20645.0,
 'sell_count_all_time': 2432864.0,
 'sell_tax': 0.0,
 'sell_volume_30d': 41309184.0,
 'sell_volume_all_time': 19360479232.0,
 'token0_address': '0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2',
 'token0_decimals': 18,
 'token0_symbol': 'WETH',
 'token1_address': '0xdac17f958d2ee523a2206206994597c13d831ec7',
 'token1_decimals': 6,
 'token1_symbol': 'USDT',
 'transfer_tax': 0.0}