[Feature Request] Allow nodes to send `NOOP` output messages

[bheijden]

🚀 Feature

Allow nodes to output a NOOP message in place of an output message.

Pitch

Currently, all nodes are forced to produce outputs at a fixed rate. This is limiting because a node may not always be able to produce an output message, and would rather want to skip sending one all together in some cases. If nodes decide to skip sending their output message, nodes downstream in the graph will wait indefinitely for a message to arrive, hence deadlocking the whole graph in sync mode.

Instead, a NOOP message should be defined that nodes can send in place of an output message. This would avoid blocking downstream nodes, but also allow upstream nodes the choice to void sending an output message.

Checklist

• I have checked that there is no similar issue in the repo (required)