blockstore: make merkle root Optional in MerkleRootMeta column #3774

Job	Run time
test	0s
check	0s
cargo-test-sbf (token/program)	7m 14s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	12m 27s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	8m 21s
cargo-test-sbf (token-upgrade/program)	9m 8s
cargo-test-sbf (feature-proposal/program)	8m 52s
cargo-test-sbf (governance/addin-mock/program, governance/program)	10m 38s
cargo-test-sbf (memo/program)	7m 21s
cargo-test-sbf (name-service/program)	6m 57s
cargo-test-sbf (stake-pool/program)	11m 40s
cargo-test-sbf (single-pool/program)	7m 51s
	1h 30m 29s

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