Do not accept falsey MR ID values

[jschwarz2030]

Documentation reference: https://learn.maproulette.org/documentation/setting-external-task-identifiers/#content

As an example, if a set of tasks each have a value of { "id": null }, the system currently accepts this as a truthy value and populates the name of the task as "null".

Proposed change: if the MR ID candidate has a falsey value, ignore it in favor of the next candidate, in this order id, @id, osmid, osm_id, name. If all candidates have falsey values, create a UUID instead.