refactor: use constant values as default values

[yangby-cryptape]

What this PR does / why we need it:

Before this PR, some values use constant values to set the default values, and some are not.

Click for example.

axon/core/consensus/src/engine.rs

Lines 71 to 79 in 4ef2f56

	let txs_root = if !txs.hashes.is_empty() {
	TrieMerkle::from_iter(txs.hashes.iter().enumerate())
	.root_hash()
	.unwrap_or_else(|err| {
	panic!("failed to calculate trie root hash for transactions since {err}")
	})
	} else {
	RLP_NULL
	};

In fact, the above code is the same as

let txs_root = { 
    TrieMerkle::from_iter(txs.hashes.iter().enumerate()) 
        .root_hash() 
        .unwrap_or_else(|err| { 
            panic!("failed to calculate trie root hash for transactions since {err}") 
        })
}; 

The only difference is avoiding some computations, so that improve a little performance.

---

In this PR:

• Unify the value assignments of those roots.
• Add documentation how these constant value come.

Which issue(s) this PR fixes:

Fixes #

Which docs this PR relation:

Ref #1318

Which toolchain this PR adaption:

No Breaking Change

Special notes for your reviewer:

NIL

CI Description

CI Name	Description
Chaos CI	Test the liveness and robustness of Axon under terrible network condition
Cargo Clippy	Run cargo clippy --all --all-targets --all-features
Coverage Test	Get the unit test coverage report
E2E Test	Run end-to-end test to check interfaces
Code Format	Run cargo +nightly fmt --all -- --check and cargo sort -gwc
Web3 Compatible Test	Test the Web3 compatibility of Axon
v3 Core Test	Run the compatibility tests provided by Uniswap V3
OCT 1-5 | 6-10 | 11 | 12-15 | 16-19	Run the compatibility tests provided by OpenZeppelin

CI Usage

Check the CI you want to run below, and then comment /run-ci.

CI Switch

• Chaos CI
• Cargo Clippy
• Coverage Test
• E2E Tests
• Code Format
• Unit Tests
• Web3 Compatible Tests
• OCT 1-5 And 12-15
• OCT 6-10
• OCT 11
• OCT 16-19
• v3 Core Tests

[KaoImin]

This PR should be refactor rather than perf.

[yangby-cryptape]

@KaoImin There is no break changes.

[KaoImin]

@yangby-cryptape need to rebase

[axon-bot]

CI tests run on commit:

• commit id: ff8900f

CI test list:

• Code Format
• Cargo Clippy
• E2E Tests
• Unit Tests
• Web3 Compatible Tests

Please check ci test results later.