In TriggerRunner, avoid reading the set of finished subtriggers when no subtrigger ever finishes.

[damccorm]

For a trigger expression, TriggerRunner tracks which of the subexpressions have finished to track which subexpression is the currently active trigger that determines when output is permitted. For some triggers (such as the default trigger) this information is not needed. The reads and writes of this data can be elided.

Imported from Jira BEAM-30. Original Jira may contain additional context.
Reported by: kenn.