NPE on error handling if user agent header does not exist

[issa-tseng]

as seen here: https://sentry.io/organizations/opendatakit/issues/1039160706/?project=1298632

relating to this line: https://github.com/opendatakit/central-backend/blob/master/lib/http/endpoint.js#L182

[NizamLZ]

@opendatakit-bot claim

[getodk-bot]

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[NizamLZ]

@opendatakit-bot claim --force

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