[Enhancement] Create a separated rule to add isNotNull for `count(FIELD)` which won't impact logical plan

**Is your feature request related to a problem?**
A followup from https://github.com/opensearch-project/sql/pull/4337#discussion_r2374473640

**What solution would you like?**
Create a separated rule to add isNotNull for `count(FIELD)`

**What alternatives have you considered?**
Keep as is.
