Unistore : Ensure Watch works in HA mode (#94120)

* Revert "Revert "Unistore : Ensure Watch works in HA mode." (#94097)"

This reverts commit 7c3fc2f261.

* make previous_resource_version nullable

* handle nil case

pkg/storage/unified/sql/testdata/mysql--resource_insert-simple.sql

@@ -5,6 +5,7 @@ INSERT INTO `resource`
         `resource`,
         `namespace`,
         `name`,
+        `previous_resource_version`,
         `value`,
         `action`
     )
@@ -14,6 +15,7 @@ INSERT INTO `resource`
         'rr',
         'nn',
         'name',
+        123,
         '[]',
         'ADDED'
     )