net: lwm2m: Ensure string termination

When writing string data to resources which are string types,
we should count in the terminating character into the data length.

Corner cases exist where LwM2M resource type is opaque but
lwm2m_get_string() or lwm2m_set_string() are used to read/write
the data. We must ensure string termination on those case, but
terminating character must not be stored in the engine buffer
or counted in the data length as this might be considered
as part of the binary data.

Fixes #59196

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