Fixed bug related to no last-read case (and processing null block number at further parts of query)

hive/db/sql_scripts/notifications_api.sql

@@ -24,7 +24,7 @@ $BODY$
 DECLARE
     __account_id INT := 0;
     __last_read_at TIMESTAMP;
-    __last_read_at_block INT;
+    __last_read_at_block hive_blocks.num%TYPE;
     __limit_block hive_blocks.num%TYPE = block_before_head( '90 days' );
 BEGIN
   __account_id = find_account_id( _account, True );
@@ -33,11 +33,13 @@ BEGIN
   FROM hive_accounts ha
   WHERE ha.id = __account_id;
 
-  SELECT hb.num INTO __last_read_at_block
-  FROM hive_blocks hb
-  WHERE hb.created_at <= __last_read_at
-  ORDER by hb.created_at desc
-  LIMIT 1;
+  --- Warning given account can have no last_read_at set, so lets fallback to the block limit to avoid comparison to NULL.
+  SELECT COALESCE((SELECT hb.num 
+                   FROM hive_blocks hb
+                   WHERE hb.created_at <= __last_read_at
+                   ORDER by hb.created_at desc
+                   LIMIT 1), __limit_block)
+    INTO __last_read_at_block;
 
   RETURN QUERY SELECT
     __last_read_at as lastread_at,