Unconnected inproc output causes pipeline to stall

I think there's an issue in Benthos - well not sure it's an issue - where an unconnected inproc output fails quietly. As in, it doesn't even trigger fallback to go to next output. Minimal repro:

input:
  generate:
    mapping: |
      root = nanoid()
    interval: 3s

output:
  fallback:
    - inproc: nonexistent
    - stdout: {}

Run this with streams mode.

Expected behaviour: there's an immediate error at the inproc output that triggers a fallback to stdout.
Actual behaviour: the benthos pipeline stalls indefintely due to backpressure while it waits for in-flight messages to be processed by inproc.

Start with the minimal YAML reproduction in the issue and run it in streams mode. Trace the inproc output and fallback behavior for the nonexistent target, then verify that the pipeline reports an immediate error and falls back to stdout instead of stalling under backpressure.