Remove and drop scripts/templates that are empty from cluster state

[jdconrad]

We deprecated the ability to add an empty stored script/template in 6.4 (#30194) with this PR. It is now safe to disallow empty stored script/templates altogether and drop any remaining from the cluster state.

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[jdconrad]

Closed by #78643