More handling null IIS rows in queries, a=chris

diff --git a/src/models/ImpressionModel.php b/src/models/ImpressionModel.php
index cd79834bb..931855d21 100644
--- a/src/models/ImpressionModel.php
+++ b/src/models/ImpressionModel.php
@@ -168,7 +168,7 @@ class ImpressionModel extends Model
     public function mostRecentThreadView($user_id, $thread_id)
     {
         $db = $this->db;
-        $sql = "SELECT MAX(UPDATE_TIMESTAMP) AS MOST_RECENT FROM " .
+        $sql = "SELECT COALESCE(MAX(UPDATE_TIMESTAMP), 0) AS MOST_RECENT FROM ".
             "ITEM_IMPRESSION_SUMMARY WHERE USER_ID = ? AND ITEM_TYPE = " .
             C\THREAD_IMPRESSION . " AND ITEM_ID = ? " .
             "AND UPDATE_PERIOD = ". C\MOST_RECENT_VIEW;
@@ -187,7 +187,7 @@ class ImpressionModel extends Model
     public function mostRecentGroupView($user_id)
     {
         $db = $this->db;
-        $sql = "SELECT MAX(UPDATE_TIMESTAMP) AS MOST_RECENT FROM " .
+        $sql = "SELECT COALESCE(MAX(UPDATE_TIMESTAMP), 0) AS MOST_RECENT FROM ".
             "ITEM_IMPRESSION_SUMMARY WHERE USER_ID = $user_id AND ITEM_TYPE = " .
             C\GROUP_IMPRESSION . " AND ITEM_ID IN (SELECT GROUP_ID " .
             "FROM USER_GROUP WHERE USER_ID = $user_id AND STATUS = ".