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/sqlite--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'
     )