tests(ci-broker.md): add scenario for empty queue

ci-broker.md

@@ -575,6 +575,25 @@ then stdout contains ""id": "xyzzy""
 ~~~
 
 
+## Processes empty event queue successfully
+
+_Want:_ CI broker does nothing, but successfully, if asked to
+process an empty event queue.
+
+_Why:_This is an important corner case for `cib queued`.
+
+_Who:_ `cib-devs`
+
+~~~scenario
+given a Radicle node, with CI configured with broker-with-triggers.yaml and adapter dummy.sh
+
+when I run ./env.sh cib --config broker-with-triggers.yaml queued
+
+when I run cibtool --db ci-broker.db run list
+then stdout is exactly ""
+~~~
+
+
 ## Handles adapter failing on a successful run
 
 _Want:_ If the adapter fails, the CI broker creates a job COB and