net: lwm2m: ignore optional resource when not implmeneted

Per LwM2M specification 7.3.2.4, "Optional Resources MAY be conveyed
in the "New Value" parameter as well; the LwM2M Client MAY ignore the
optional resources it doesn't support."

Update TLV/JSON writer to ignore error when object fields are not
found (treated as optional resource). This will allow the resources
supported being written.

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>
[michael.scott@linaro.org: re-worked patch post addition of CREATE
operation.]
Signed-off-by: Michael Scott <michael.scott@linaro.org>