fix: stop-only metadata no longer drops options for up/down

A previous change extended IsEmpty() to include `Stop == nil`. For a
provider that advertises only a `stop` block, this caused
commandMetadataIsEmpty to be false in setupPluginCommand, which made
the option-forwarding filter reject every key — silently dropping
all provider options on `up` and `down`.

The Stop-presence signal lives independently in stop.go and the
"stop" case of setupPluginCommand, so reverting the IsEmpty check
to its original semantics is sufficient.

Addresses PR #13779 review feedback.

Signed-off-by: Guillaume Lours <glours@users.noreply.github.com>