Omit GeoJSON properties with null values

[kylebarron]

In geoarrow/geoarrow-rs#588 I got a bug report about a GeoJSON file that failed to read. The issue here is a column that's numeric for the first N rows but then is null for some row N + 1. Currently the GeoJSON reader coerces a null value to the string "null":

geozero/geozero/src/geojson/geojson_reader.rs

Lines 220 to 221 in 3378dda

	// Null, Array(Vec<Value>), Object(Map<String, Value>)
	_ => processor.property(i, key, &ColumnValue::String(&value.to_string()))?,

This is hard to work with because in my processor I'd have to check every input string value (from any geozero reader, for any column) against the string "null" and manage type conversions. E.g. if the first few rows of a column are the string "null" but then I see an f64 value, I need to be able to coerce that column to a Float64Array (which in Arrow has its own null bitmask).

I'd argue that the simplest approach in this specific case is to do a no-op for GeoJSON null values. That {"a": 1, "b": null} is equivalent to {"a": 1}. That seems like a much smaller change than, say, adding a ColumnValue::Null variant. But interested to hear others' thoughts.

[michaelkirk]

I think this is a mostly uncontroversial improvement, and so we should merge it.

Beyond that, I have thoughts, but no conclusions, about introducing an explicit Null type into geozero...

Even though I think your change is mostly uncontroversially an improvement, I'm going to let it sit for a couple days before merging in case anyone else wants to weigh in.