`tokens` on a `Nullable(String)` and similar string splitting functions should work

### Company or project name

ClickHouse

### Use case

I read an external data, so automatically get Nullable types. Then applying `tokens` requires a cast.

### Describe the solution you'd like

It is okay if they will output an empty array on NULL input.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_