[C++] `SelectKUnstable` can crash on empty inputs

[taepper]

Describe the bug, including details regarding any error messages, version, and platform.

If an input Table or ChunkedArray is empty, the SelectKthInternal functions in TableSelector and ChunkedArraySelector have the following early return:

      if (num_rows == 0) {                                                                                                                                                                          
        return Status::OK();                                                                                                                                                                     
      }                                                                                                                                                                                               

Crucially, output_ is never set in these cases. Therefore, SelectKUnstable's make_array() call:

  Result<std::shared_ptr<Array>> SelectKUnstable(const Datum& datum, ...) {                        
    ARROW_ASSIGN_OR_RAISE(Datum result,                                                            
                          CallFunction("select_k_unstable", {datum}, &options, ctx));               
    return result.make_array();
  }                                                                                                 

will fail this DCHECK_EQ:

  std::shared_ptr<Array> Datum::make_array() const {                                                                                                                                                  
    DCHECK_EQ(Datum::ARRAY, this->kind());
    return MakeArray(std::get<std::shared_ptr<ArrayData>>(this->value));                                            
  }                                                                                                                                                                                            

In release builds. std::get will crash with a std::bad_variant_access

This can also be reproduced in python code by calling select_k_unstable with an empty ChunkedArray or Table respectively

Component(s)

C++

[Adarsh-Me]

Hi, I'd like to work on this issue. Is it okay if I take this up?

[taepper]

take