net: lwm2m: Made pmin and pmax attributes optional

Made the LwM2M engine checks for pmin and pmax optional to adhere to
the LwM2M specificattion. We now first check that pmin and pmax are
actually set. Also changed the default CONFIG values for the attributes
pmin and pmax to 0 to indicate that they are not active by default.

Fixes #34329.

Signed-off-by: Maik Vermeulen <maik.vermeulen@innotractor.com>