chore: make `MultiPointBuilder::push_null()` public

[JosiahParry]

PointBuilder has a push_null() however, the MultiPointBuilder does not. It would be great if the multipoint builder had a way to add null values to it.

[kylebarron]

First,

It would be great if the multipoint builder had a way to add null values to it.

You can push null values by passing None to push_geometry, push_line_string, or push_multi_line_string

---

Secondly, I think the goal was for push_null to be exposed through the builder trait:

geoarrow-rs/rust/geoarrow-array/src/trait_.rs

Lines 512 to 538 in b7f8864

	/// A trait describing a mutable geometry array; i.e. an array whose values can be changed.
	///
	// Note: This trait is not yet publicly exported from this crate, as we're not sure how the API
	// should be, and in particular whether we need this trait to be dyn-compatible or not.
	pub(crate) trait GeoArrowArrayBuilder: Debug + Send + Sync {
	/// Returns the length of the array.
	fn len(&self) -> usize;
	/// Returns whether the array is empty.
	fn is_empty(&self) -> bool {
	self.len() == 0
	}
	/// Push a null value to this builder.
	fn push_null(&mut self);
	/// Push a geometry to this builder.
	#[allow(dead_code)]
	fn push_geometry(
	&mut self,
	geometry: Option<&impl GeometryTrait<T = f64>>,
	) -> GeoArrowResult<()>;
	/// Finish the builder and return an [`Arc`] to the resulting array.
	#[allow(dead_code)]
	fn finish(self) -> Arc<dyn GeoArrowArray>;
	}

but as noted, there are hard decisions about the public API there.

---

If we were to make the underlying push_null method public, we'd want to make it consistent across all builders. I'm also not sure if having both a push_null method and hopefully a future public builder trait with the same push_null name would be problematic at all

[JosiahParry]

I am using the None variant right now but it requires None::<&MultiPoint<f64>> which was somewhat difficult to figure out over a simple .push_null()

[kylebarron]

Do you know if having a push_null method will interact with a future push_null trait method?