avoid trying to set the admin email to nil in data migration.

src/metabase/db/migrations.clj

@@ -85,4 +85,5 @@
 ;; Populate the initial value for the `:admin-email` setting for anyone who hasn't done it yet
 (defmigration set-admin-email
   (when-not (setting/get :admin-email)
-    (setting/set :admin-email (db/sel :one :field ['User :email] (k/where {:is_superuser true :is_active true})))))
+    (when-let [email (db/sel :one :field ['User :email] (k/where {:is_superuser true :is_active true}))]
+      (setting/set :admin-email email))))