Bluetooth: Controller: Fix coverity issue for cis == NULL

Coverity was unhappy that
`struct ll_conn_iso_stream *cis = NULL;` was never assigned to
a non-NULL value, which is due to the assignment being
guarded by `#if defined(CONFIG_BT_CTLR_CONN_ISO)`.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>