You could add '--- All Stores ---' to your list. Here is the query I would use to populate the drop-down. S.O. answer here
SELECT
store as store_name
FROM
(
Select Distinct
store
From
Table
UNION ALL
SELECT
'--- All Stores ---' AS store
)
ORDER BY
store_name
Next the query that uses the drop down
Select
*
FROM
Table
WHERE
(store in ({{store_name}}) or '{{store_name}}' LIKE '--- All Stores ---')
Now if you select '--- All Stores ---' you should get all of the stores ๐