refactor(chain)!: Change tx_last_seen to `Option<u64>` #5601

Job	Run time
Rust fmt	6s
clippy_check	59s
Build and test (stable, true, --no-default-features)	3m 38s
Build and test (stable, true, --all-features)	3m 36s
Build and test (1.63.0, --no-default-features)	5m 14s
Build and test (1.63.0, --all-features)	5m 26s
Check no_std	25s
Check WASM	1m 0s
	20m 24s

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