read-session should return nil when there is no value

this is to be consistent with most all other implementations of
ring's SessionStore that i've seen

src/aging_session/memory.clj

@@ -59,7 +59,7 @@
     (swap! session-map sweep-entry event-fns key)
     (when (and refresh-on-read (contains? @session-map key))
       (swap! session-map assoc-in [key :timestamp] (now)))
-    (get-in @session-map [key :value] {}))
+    (get-in @session-map [key :value]))
 
   (write-session [_ key data]
     (let [key (or key (str (UUID/randomUUID)))]

test/aging_session/event_test.clj

@@ -10,7 +10,7 @@
     (let [as (aging-memory-store :events [(event/expires-after 1)])]
       (write-session as "mykey" {:foo 1})
       (. Thread (sleep 1500))
-      (is (= (read-session as "mykey") {})))))
+      (is (nil? (read-session as "mykey"))))))
 
 (deftest session-expiry-by-sweep
   (testing "Test session expiry sweep."
@@ -43,6 +43,6 @@
                :refresh-on-read true
                :sweep-every 1
                :sweep-delay 1000)]
-      (is (= (read-session as "foo") {}))
+      (is (nil? (read-session as "foo")))
       ; read again to trigger the sweep
-      (is (= (read-session as "foo") {})))))
+      (is (nil? (read-session as "foo"))))))

test/aging_session/memory_test.clj

@@ -7,7 +7,7 @@
 (deftest basic-read-empty
   (testing "Test session reads."
     (let [as (aging-memory-store)]
-      (is (= (read-session as "mykey") {})))))
+      (is (nil? (read-session as "mykey"))))))
 
 (deftest basic-write
   (testing "Test session writes and reads."
@@ -22,7 +22,7 @@
     (let [as (aging-memory-store)]
       (write-session as "mykey" {:a 1})
       (delete-session as "mykey")
-      (is (= (read-session as "mykey") {})))))
+      (is (nil? (read-session as "mykey"))))))
 
 (deftest timestamp-on-creation
   (testing "Test the behaviour where each entry's timestamp is set only on session creation."