title: "proposer: return optional block"

doc:
  - audience: Node Dev
    description: |
      The `ProposerInterface` trait now returns an optional `Proposal`, allowing
      for no block to be created. This is a breaking change that only impacts custom
      `ProposerInterface` implementations. The change allows more flexibility in choosing
      when to create blocks.

crates:
  - name: "cumulus-client-consensus-aura"
  - name: "cumulus-client-consensus-proposer"