Labeled metrics with dynamic labels result in SQL error due to UNNEST(NULL)

[badboy]

originally reported by @MatthewTighe in chat.

Example metric: recent_synced_tabs.recent_synced_tab_shown

Looker link

Error:

Query execution failed: - Values referenced in UNNEST must be arrays. UNNEST contains expression of type INT64 at [11:18]

In the generated SQL I see:

LEFT JOIN UNNEST(NULL) AS metrics__metrics__labeled_counter__recent_synced_tabs_recent_synced_tab_shown ON metrics.document_id = metrics.document_id

I think this corresponds to one of these generated LookML parts:

• https://github.com/mozilla/looker-hub/blob/824bf61c034522399efbbafe566fd5d5fef10831/fenix/explores/metrics.explore.lkml#L317-L320
• https://github.com/mozilla/looker-hub/blob/main/fenix/views/metrics_table.view.lkml#L2091-L2097

Metrics with static labels, such as engine_tab.kills seem to work fine (Looker link).

cc @scholtzan

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