handle nil last-updated times by setting :default to epoch (#21456)

* handle unset last-updated times
* magic number -> zoned-time-epoch
* test that slack cache gets filled when
slack-channels-and-usernames-last-update is nil.

- remove a comment

src/metabase/integrations/slack.clj

@@ -42,11 +42,14 @@
   :visibility :internal
   :type :json)
 
+(def ^:private zoned-time-epoch (t/zoned-date-time 1970 1 1 0))
+
 (defsetting slack-channels-and-usernames-last-updated
   "The updated-at time for the [[slack-cached-channels-and-usernames]] setting."
   :visibility :internal
   :cache?     false
-  :type       :timestamp)
+  :type       :timestamp
+  :default    zoned-time-epoch)
 
 (defsetting slack-files-channel
   (deferred-tru "The name of the channel to which Metabase files should be initially uploaded")

test/metabase/api/slack_test.clj

@@ -33,7 +33,8 @@
             (is (= [] (slack/slack-cached-channels-and-usernames)))))))
 
     (testing "The Slack files channel setting can be set by an admin, and the leading # is stripped if it is present"
-      (mt/with-temporary-setting-values [slack-files-channel nil]
+      (mt/with-temporary-setting-values [slack-files-channel                       nil
+                                         slack-channels-and-usernames-last-updated nil]
         (with-redefs [slack/channel-exists? (constantly true)]
           (mt/user-http-request :crowberto :put 200 "slack/settings" {:slack-files-channel "fake-channel"})
           (is (= "fake-channel" (slack/slack-files-channel)))

test/metabase/integrations/slack_test.clj

@@ -236,3 +236,7 @@
               (is (= false (slack/valid-token? "abc")))
               (is (= {} (mt/summarize-multipart-email #"Your Slack connection stopped working.")))
               (is (slack/slack-token-valid?)))))))))
+
+(deftest slack-cache-updated-at-nil
+  (tu/with-temporary-setting-values [slack-channels-and-usernames-last-updated nil]
+    (is (= (var-get #'slack/zoned-time-epoch) (slack/slack-channels-and-usernames-last-updated)))))