When find_event_pipeline cannot find an complex cadence matches, it crashes

[texadactyl]

Describe the bug

Given a bogus source name, find_event_pipeline() tries to derive something from a nil internal table, leading to a crash with a traceback ending in this message: "ValueError: 1 is not in list". :(

To Reproduce

Steps to reproduce the behavior:

1. Run find_event_pipeline() on the Voyager 2020 set but say on_source_complex_cadence='Voyager-42'.
2. See error.

Expected behavior

When the entire derived cadence is all 0s, then

• DIsplay a message to this effect.
• Return to caller with None (no matches, no candidates).