handle `push_null` in the unknown geometry builder #2174

Job	Run time
build-benchmarks	3m 40s
Lint and Test	6m 57s
check-features-with-external-dependencies (-F gdal -F gdal/bindgen)	1m 10s
check-features	12s
check-features-with-external-dependencies (-F geos)	33s
check-features (-F csv)	12s
check-features-with-external-dependencies (-F proj)	43s
check-features (-F flatgeobuf)	20s
check-features (-F flatgeobuf_async)	34s
check-features (-F ipc_compression)	27s
check-features (-F parquet)	30s
check-features (-F parquet_async)	30s
check-features (-F parquet_compression)	23s
check-features (-F polylabel)	12s
check-features (-F postgis)	45s
check-features (-F rayon)	20s
	17m 28s

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