Gas tracker tooltip shows 'null' value when backend returns 'null'

[yvaskov]

Description

Widget shows null:

Top bar shows nothing:

Backend sample response for /api/v2/stats:

{
    "average_block_time": 16606.0,
    "coin_price": null,
    "coin_price_change_percentage": null,
    "gas_price_updated_at": null,
    "gas_prices": {
        "average": null,
        "fast": null,
        "slow": null
    },
    "gas_prices_update_in": 30000,
    "gas_used_today": "50897733280",
    "market_cap": "0",
    "network_utilization_percentage": 31.607558,
    "static_gas_price": null,
    "total_addresses": "158901510",
    "total_blocks": "10232689",
    "total_gas_used": "0",
    "total_transactions": "295253201",
    "transactions_today": "554793",
    "tvl": null
}

Proposal

Return 'N/A' instead of 'null'

Version

v1.21.0-alpha.1