[C++][Arrow] StructBuilder::AppendNull should call the AppendNull method of its Child

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

```C++
/// cpp/src/arrow/array/builder_nested.h:774~792

/// \brief Append a null value. Automatically appends an empty value to each child
/// builder.
Status AppendNull() final {
for (const auto& field : children_) {
ARROW_RETURN_NOT_OK(field->AppendEmptyValue());
}
return Append(false);
}

/// \brief Append multiple null values. Automatically appends empty values to each
/// child builder.
Status AppendNulls(int64_t length) final {
for (const auto& field : children_) {
ARROW_RETURN_NOT_OK(field->AppendEmptyValues(length));
}
ARROW_RETURN_NOT_OK(Reserve(length));
UnsafeAppendToBitmap(length, false);
return Status::OK();
}
```

If data is written in this way, the `null_count` in the child array will be inconsistent when read back.

### Component(s)

C++