last_value support for Variant type

### Company or project name

_No response_

### Use case

https://fiddle.clickhouse.com/7cf75f16-c433-4c33-b865-99ce25324ef6

Coalescing merge tree is using last_value as aggregate function, so it's needed for proper work of arrays/other complex column types in Coalescing merge tree
https://github.com/ClickHouse/ClickHouse/issues/84015#issuecomment-3126914921
https://github.com/ClickHouse/ClickHouse/pull/79344

### Describe the solution you'd like

last_value will skip "Nullable(Nothing)" values of Variant type

### Describe alternatives you've considered

_No response_

### Additional context

_No response_