fix(cli): allow empty strings and falsy values in config set

`hermes config set KEY ""` and `hermes config set KEY 0` were rejected
because the guard used `not value` which is truthy for empty strings,
zero, and False. Changed to `value is None` so only truly missing
arguments are rejected.

Closes #4277

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hermes_cli/config.py

@@ -2040,7 +2040,7 @@ def config_command(args):
     elif subcmd == "set":
         key = getattr(args, 'key', None)
         value = getattr(args, 'value', None)
-        if not key or not value:
+        if not key or value is None:
             print("Usage: hermes config set <key> <value>")
             print()
             print("Examples:")