json: exclude None in output, and sort keys

These are both size/performance enhancements. Not including 'None'
values will reduce document sizes on-disk and over network, particularly
for intermediate objects. Sorting by key should improve compression
ratios across multiple documents, both on-disk (gzip) and in
elasticsearch itself:

  https://www.elastic.co/guide/en/elasticsearch/reference/current/tune-for-disk-usage.html#_put_fields_in_the_same_order_in_documents

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