useEventList()/useMetadataList() send a REQ that can only return nothing for an empty input

Two hooks derive `limit` from the length of their input array:

- `src/lib/stores/useEventList.ts:21` — `{ ids, limit: ids.length }`
- `src/lib/stores/useMetadataList.ts:21` — `{ kinds: [0], authors: pubkeys, limit: pubkeys.length }`

Given an empty array this sends `["REQ", subId, {"ids":[],"limit":0}]`.
rx-nostr's `normalizeFilter` keeps `limit: 0` (`rx-req.ts:229`), and `use()`'s
guard `filter(({ filters }) => filters.length > 0)` counts *filters*, not ids, so
the REQ goes out to every relay. It can only ever return nothing.

An empty input is a normal thing for a caller to hold briefly — an id list derived
from another query that has not resolved yet, or a contact list that is still
loading.

Determined by code reading, with the `normalizeFilter` behaviour checked in the
installed rx-nostr source.

## Direction

Short-circuit on an empty input: return the empty result without sending a REQ.