StructBuilder::append_null should recursively append_null to children

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

At the moment, it's impossible to generically `append_null` to a StructBuilder, because each nested column must also be appended to. Going through `field_builder` is not an option, because it requires knowing the type of the field ahead of time.

**Describe the solution you'd like**

Calling `append_null` on a StructBuilder should probably always `append_null` to all children with exactly n-1 slots. Alternatively, we could have a second function, called `append_nulls` or something similar.

**Describe alternatives you've considered**

If there was a way to generically iterate over the field builders, callers could implement it themselves, using a match on the type of the builder.

Start by locating StructBuilder::append_null and the child field_builder APIs. Trace how struct children are currently appended, then verify the chosen behavior for recursively appending nulls, including nested children and the expected number of slots.

Written by the indexing model from the issue text.