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/postgres--resource_version_insert-single path.sql

@@ -7,6 +7,6 @@ INSERT INTO "resource_version"
     VALUES (
         '',
         '',
-        1
+        2
     )
 ;