The v3.4.1 command filter used bot.get_context() to detect commands,
but an empty-string prefix ('') matched EVERY message, causing
ctx.valid to return True for any message whose first word happened
to be a registered command. With enough built-in commands (help,
ping, info, set, etc.) this silently dropped most or all relay
traffic (total_messages stayed at 0).
Fix: require ctx.prefix to be truthy (non-empty) before skipping
a message as a command. Empty-prefix matches are now ignored,
so only messages with a real prefix (!, @bot, etc.) are filtered.
Also re-added the empty-prefix warning to 'wh debug' output so
users know to clean up their bot prefix configuration.
