Bluetooth: Add comment clarifying condition check in bt_le_adv_start

It is valid to call set_ad with NULL data as long as length is 0.
Improve code comment for unusual condition check explaning why it
is correct.

Change-Id: I29440fb04747f6df06dc4df466eb43e160ff3be4
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>