undefined check for map array fn

[GermaVinsmoke]

Fixes #840

@hellais It'll make the time granularity list empty in case of invalid date (year in bug)

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Name	Status	Preview	Comments	Updated
explorer	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Feb 24, 2023 at 6:20PM (UTC)

[majakomel]

I'll close this as it was fixed in another commit (https://github.com/ooni/explorer/commit/329cc9ac9b0277cc625ed29fcc50046adb5daccf)