Fix: usage of postgresql -> postgres for db_type (#10120)

### What problem does this PR solve?

This PR fixes incorrect naming for PostgreSQL usage by replacing all
instances of `postgresql` with the correct `postgres` in the `db_type`
field. This resolves potential configuration errors and ensures
consistency when specifying the database type.

Also fixed handling of None for `get_queue_length`

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: cucusenok <BP-116: updated readme.md>

api/apps/canvas_app.py

@@ -332,7 +332,7 @@ def test_db_connect():
         if req["db_type"] in ["mysql", "mariadb"]:
             db = MySQLDatabase(req["database"], user=req["username"], host=req["host"], port=req["port"],
                                password=req["password"])
-        elif req["db_type"] == 'postgresql':
+        elif req["db_type"] == 'postgres':
             db = PostgresqlDatabase(req["database"], user=req["username"], host=req["host"], port=req["port"],
                                     password=req["password"])
         elif req["db_type"] == 'mssql':