Don't bother hitting the DB if we got everything from cache (#3376)

Yet another finding where we hit the DB with a useless query. [skip ci]

federationapi/storage/cache/keydb.go

@@ -46,6 +46,10 @@ func (d *KeyDatabase) FetchKeys(
 			delete(requests, req)
 		}
 	}
+	// Don't bother hitting the DB if we got everything from cache.
+	if len(requests) == 0 {
+		return results, nil
+	}
 	fromDB, err := d.inner.FetchKeys(ctx, requests)
 	if err != nil {
 		return results, err