When the Slack API returned no channels (due to upstream failures),
json.MarshalIndent(nil) produced "null" which was written to the
cache file. On subsequent startups, json.Unmarshal("null") into a
slice succeeds with no error, producing an empty slice that was
treated as valid cached data — permanently serving empty results
until the cache TTL expired.
Three defense-in-depth guards:
- Don't write cache file when channel list is empty
- Treat empty/null unmarshaled channels cache as invalid (refetch)
- Same empty-cache guard for users cache (consistency)
George Bashi