Bluetooth: controller: Fix initial empty flag for peripheral

Fix to avoid an assertion when processing an acknowledgment
in the first connection event in peripheral role.

Ensure that empty flag reflects the state of the Tx queue,
as a peripheral the first connection event has no prior PDU
transmitted, an incorrect acknowledgment by peer should
not dequeue a PDU that has not been transmitted on air.

Relates to assert added in commit 2bfaadff ("Bluetooth:
controller: Add Tx fragmentation assertion").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>