Always initialize 'doc' variable as a Hash

In the `UnifiedSearchPresenter`, the current behaviour assumed that the `fields` key returned for an Elasticsearch document is always a hash.

However, if a request is made for only a small number of fields, and the returned document contains none of them, then the `fields` key is nil. This causes an exception when we attempt to assign to the `_metadata`  key, as `doc` is nil.

This fixes this by using the double-pipe convention to fallback to an empty hash, in the case that the value of `fields` is nil.
A closed hash (`{}`) is common in these circumstances, see line 20.