net: lwm2m: Clear out the timestamp when we start registration

There was a corner case that last registration timestamp was used
to detect if we can try UPDATE instead of full registration.

When timestamp was not cleared and DNS resolving failed, it might
cause engine to skip the resolving and continue retrying an UPDATE
message until timeout.

Fixes #54504

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>