fix: no chain_root in 1st block of the mmr activated epoch

[yangby-cryptape]

What problem does this PR solve?

Since CKB enabled the MMR feature through version bits with following code:

ckb/verification/contextual/src/contextual_block_verifier.rs

Lines 567 to 569 in 3d674d5

	let mmr_active = self
	.context
	.versionbits_active(DeploymentPos::LightClient, self.parent);

ckb/spec/src/consensus.rs

Line 1031 in 3d674d5

versionbits.get_state(parent, cache, indexer)

the extension field of the first block in the MMR activated epoch, doesn't contain the chain root.

So, when check the extension fields, the first block in the MMR activated epoch should be skipped.

Ref: nervosnetwork/ckb-light-client#163

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code ci-runs-only: [ quick_checks,linters ]

Release note

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