Bluetooth: GATT: Fix issue with 0/NULL for bt_gatt_attr_read

The bt_gatt_attr_read function did not properly handle cases
where value is NULL and/or value_len is 0, or bad mixes of the
two.

The important part is here is that we do not perform the pointer
arithmetic on NULL and/or call memcpy with a NULL src with a
non-0 length.

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