net: lwm2m: Fix attribute sanity check

Validate the respective attribute parameters only if both are provided
at given level. This prevents from comparing for instance unset pmax
value (equal to 0) with some new pmin value sent by the server.

Additionally, fix the sanity check for the `pmax` value set on observer,
after fetching the attribute value at all levels (including the default
value at from the server object). Instead of using wrong max(pmin, pmax)
formula, set the pmax value only if it's a valid one (>= pmin),
otherwise ignore the value and set it to 0. This makes the notification
engine ignore the pmax value set for observation.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>