Possibly empty DB updates feed produces error

[flimzy]

--- FAIL: TestCouch23 (78.90s)
    test.go:410: Connecting to http://admin:abc123@couch2.3.1:5984/ ...
    test.go:417: Connecting to http://couch2.3.1:5984/ ...
    --- FAIL: TestCouch23/DBUpdates (60.01s)
        --- FAIL: TestCouch23/DBUpdates/RW (0.00s)
            --- FAIL: TestCouch23/DBUpdates/RW/Admin (60.01s)
                updates.go:44: Unexpected failure: 502/kivik: unexpected JSON token "last_seq" in feed response, expected `db_name` or `results`
FAIL
FAIL	github.com/go-kivik/kivik/v4/couchdb/test	78.920s

My theory is this happens when the db updates feed is empty. last_seq is a valid top-level key—are we getting it when there are no results? Needs to be verified, and then possiblyfixed.