Aggs: The nested aggregation should work on unmapped fields

[jpountz]

When a field is unmapped, aggregations try to behave as if the field existed but did not contain any value. The nested aggregation is an exception to this rule and fails if it is asked to run on a path that is not mapped, I think we should fix it to be consistent?

See https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/elasticsearch/f0VYoHs1SpU/dx_iZW-U65kJ for more context.