Preserve null signal values on roundtrip (de)serialization #811

Job	Run time
build-vegafusion-jupyter-packages	5m 7s
build-vegafusion-packages	1m 2s
build-vegafusion-python-embed-linux-64	2m 51s
build-vegafusion-python-embed-linux-arm64	11m 17s
build-vegafusion-python-embed-osx-64	19m 56s
build-vegafusion-python-embed-osx-arm64	19m 10s
build-vegafusion-python-embed-win-64	17m 14s
build-vegafusion-server-linux-64	9m 52s
build-vegafusion-server-linux-arm64	2m 11s
build-vegafusion-server-osx-64	21m 52s
build-vegafusion-server-osx-arm64	22m 6s
build-vegafusion-server-win-64	16m 20s
build-vegafusion-wasm	3m 21s
check-rust	7m 0s
test-rust (ubuntu-20.04)	25m 22s
test-rust (windows-2022)	35m 40s
test-rust (macos-11)	57m 4s
test-vegafusion-python-linux-64	7m 11s
test-vegafusion-jupyter-linux-64	24m 27s
test-vegafusion-python-win-64	11m 34s
test-vegafusion-python-osx-64	11m 54s
	5h 32m 31s

Provide feedback