Empty day messages breaking history chain

As dotemacs pointed out on slack:
```
Not sure what’s going on, but on https://clojurians-log.clojureverse.org/datascript/2019-04-10
if you click on the < button, which links to: https://clojurians-log.clojureverse.org/datascript/2019-03-18
I get 404.
```

My response to that:
```
Just repl'd in to check whats the issue, somehow there are no messages for that day but it still shows up as a valid date. This breaks our frontend rendering.
user=> (some #{"2019-03-18"} (map first (q/channel-days (db) "datascript")))
"2019-03-18"
user=> (q/channel-day-messages (db) "datascript" "2019-03-18")
()
As a temporary workaround you can use these dates so you can keep traversing the history :slightly_smiling_face:
user=> (take 5 (drop-while #(not= % "2019-03-18") (map first (q/channel-days (db) "datascript"))))
("2019-03-18" "2019-02-28" "2019-02-16" "2019-02-15" "2019-02-09")
https://clojurians-log.clojureverse.org/datascript/2019-02-28
```

This was a temporary fix, but we need to look into why an empty day is being cached