The query term "key>" can be used to search for records that have any
value for the key. This optimizes the query with a separate SQL
expression and adds tests for this behavior.

Change-Id: I5e2734734e0911a3ed12c87e7ada776ad309a90d
Reviewed-on: https://go-review.googlesource.com/36590Reviewed-by: Russ Cox <rsc@golang.org>