Gas tracker behaviour in case of null gas prices from backend

[yvaskov]

Description

When backend returns null for all 3 gas values, we should:

• Display a disclaimer on Gas tracker page
• Hide gas tracker widget on homepage

This behaviour should occur only with 3 nulls:

"gas_prices": {
    "average": null,
    "fast": null,
    "slow": null
}

In case when backend returns an update with nulls (after providing valid values in the first place), N/A should be displayed instead.

Text for empty state:
"No data available yet"