Method is_fully_null

cpp/src/arrow/util/rle_encoding_internal.h:867–867

  constexpr size_type total_read() const noexcept { return values_read_ + null_read_; }

  constexpr bool is_fully_null() const noexcept { return values_remaining() == 0; }

  constexpr bool is_done() const noexcept { return total_remaining() == 0; }