(PE-36120) Do not kill connections when no migrations are pending

Previous behavior would cause compilers to be disconnected from
postgresql ever time the primary's pe-puppetdb was restarted. This was because the
restart invokes the migration code path, which, in PE with a
migrator-account set leads to connection blocking even when the
migration itself is ultimately a noop.

Now it tests whether any migrations are pending before calling
call-with-connections-blocked-during-migration. This change also means
that update-schema will return nil in this case, and initialize-schema
will return false, which is a change from previous noop migrations where
initialize-schema would have returned true.

src/puppetlabs/puppetdb/scf/migrate.clj

@@ -2617,16 +2617,17 @@
                      tables))]
     (if-not write-user
       (migrate)
-      (call-with-connections-blocked-during-migration
-       db-name
-       (distinct [read-user write-user])
-       (fn []
-         ;; So new tables, etc. are owned by the write-user
-         (jdbc/do-commands (str "set role " (jdbc/double-quote write-user)))
-         (try!
-          (migrate)
-          (finally
-            (jdbc/do-commands (str "set role " (jdbc/double-quote orig-user))))))))))
+      (when-not (empty? (pending-migrations))
+        (call-with-connections-blocked-during-migration
+         db-name
+         (distinct [read-user write-user])
+         (fn []
+           ;; So new tables, etc. are owned by the write-user
+           (jdbc/do-commands (str "set role " (jdbc/double-quote write-user)))
+           (try!
+            (migrate)
+            (finally
+              (jdbc/do-commands (str "set role " (jdbc/double-quote orig-user)))))))))))
 
 (defn initialize-schema
   "Ensures the database is migrated to the latest version, and returns