Bluetooth: L2CAP: Set NULL callback for PDUs

It was not being set, and thus if the user_data contained garbage from
before, then conn.c would attempt to call that garbage.

Static channels don't have this issue, as every "SDU" fits into one PDU.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
Co-authored-by: Huajiang Zheng <nxf88597@lsv051208.swis.nl-cdc01.nxp.com>