SIGNALTTY_SOCKET= resolved the socket to an empty path, and an empty
SIGNALTTY_AGENTS_DIR or SIGNALTTY_PLUGIN_DIR silently loaded nothing
instead of the config-dir default. Empty now means unset, like the XDG
variables. The CLI's integration status reads the agents dir through
paths::agents_dir instead of its own copy of the override.

Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>