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/data/resource_version_insert.sql

@@ -8,6 +8,6 @@ INSERT INTO {{ .Ident "resource_version" }}
     VALUES (
         {{ .Arg .Group }},
         {{ .Arg .Resource }},
-        1
+        2
     )
 ;