Make sure timeseries data shows nulls instead of zero when data is not available

[murderteeth]

reported from discord, https://discord.com/channels/734804446353031319/1334168008192430140

Hi people, just to let you know kong is missing some values (which seem to be populated with zeros, would rather have received NaNs and would have detected the issue more easily):

query Tvls($chainId: Int!, $address: String, $limit: Int) {
    tvls(chainId: $chainId, address: $address, limit: $limit) {
        time
        priceUsd
    }
}

This query returns zeros in priceusd.

You can test with 0x4d3ceBA4349ADB06d2De8EBD2F9320A61303aD81
but it's true for many vaults.
Would propose to keep NaNs in the db, if impossible to have a proper measurement of the price, thank you

[jorphex]

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