Fix BigQuery select with null values

[leszko]

Before this PR, if BigQuery returns null in some of the columns, then a call to /data/views/query returns an error:

"bigquery error: error reading query result: bigquery: NULL cannot be assigned to field PlaytimeMins of type float64"
After this change, it will return the following:

[
    {
        "viewCount": 0,
        "playtimeMins": 0,
        "ttffMs": 0,
        "rebufferRatio": 0,
        "errorRate": 0,
        "exitsBeforeStart": 0
    }
]

Related to https://linear.app/livepeer/issue/PS-191/grafana-engagement-data-issues, though it does no solve the root cause of that issue.

[linear]

PS-191 Grafana Engagement Data Issues

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (590d298) 28.00000% compared to head (204cf9c) 28.00000%.

Additional details and impacted files

@@              Coverage Diff              @@
##                main        #178   +/-   ##
=============================================
  Coverage   28.00000%   28.00000%           
=============================================
  Files              4           4           
  Lines            325         325           
=============================================
  Hits              91          91           
  Misses           223         223           
  Partials          11          11           

Files	Coverage Δ
views/bigquery.go	49.32432% <100.00000%> (ø)

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Files	Coverage Δ
views/bigquery.go	49.32432% <100.00000%> (ø)