L2 Node: Instant confirmation empty blocks can include delayed txns

What

Fix delayed inbox message inclusion logic when instant confirmations are enabled.

Why

Delayed inbox messages can never be included in empty blocks.

How

state.selected_delayed_txns and state.validated_txns are only manipulated by instant confirmation logic. We want to verify that:

1. Standard block production is used if IC disabled
2. Always use accumulated transactions if delayed txns have been selected through instant confirmations
3. When instant confirmation views a soon-to-be produced block as empty, force delayed txns selection

⚠️ I am not convinced by 3. as it requires extra durable storage accesses. I'm wondering if we'd rarther keep the current logic and require at least one common transaction to include delayed inbox transactions.