add nil expr for summarise and mutate (#816) #371

Job	Run time
NIF 2.15 - aarch64-apple-darwin (macos-11 \| default)	34m 19s
NIF 2.15 - aarch64-unknown-linux-gnu (ubuntu-20.04 \| default)	9m 45s
NIF 2.15 - aarch64-unknown-linux-musl (ubuntu-20.04 \| default)	9m 43s
NIF 2.15 - x86_64-apple-darwin (macos-11 \| default)	34m 4s
NIF 2.15 - x86_64-pc-windows-gnu (windows-2022 \| default)	19m 17s
NIF 2.15 - x86_64-pc-windows-gnu (windows-2022 \| legacy_cpu)	19m 35s
NIF 2.15 - x86_64-pc-windows-msvc (windows-2019 \| default)	18m 24s
NIF 2.15 - x86_64-pc-windows-msvc (windows-2019 \| legacy_cpu)	18m 11s
NIF 2.15 - x86_64-unknown-freebsd (ubuntu-22.04 \| default)	12m 58s
NIF 2.15 - x86_64-unknown-freebsd (ubuntu-22.04 \| legacy_cpu)	13m 18s
NIF 2.15 - x86_64-unknown-linux-gnu (ubuntu-20.04 \| default)	7m 55s
NIF 2.15 - x86_64-unknown-linux-gnu (ubuntu-20.04 \| legacy_cpu)	8m 13s
NIF 2.15 - x86_64-unknown-linux-musl (ubuntu-20.04 \| default)	9m 22s
	3h 35m 4s

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