Bluetooth: Allow for NULL data in mgmt_pending_add

Since index is in mgmt_hdr it is possible to have mgmt command with
no parameters that still needs to add itself to pending list.

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>