[Prover][trivial] Fix generation of `IsValid` for variants with no fields #70947

Job	Run time
file_change_determinator	14s
rust-doc-tests	0s
rust-consensus-only-unit-test	0s
rust-unit-tests	0s
rust-consensus-only-smoke-test	0s
rust-lints	3m 37s
rust-cargo-deny	1m 42s
general-lints	27s
rust-cryptohasher-domain-separation-check	0s
rust-smoke-tests	0s
rust-check-merge-base	1m 20s
rust-targeted-unit-tests	10m 9s
rust-build-cached-packages	0s
	17m 29s

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