net: lwm2m: Allow string and opaque data types to be empty

When string and opaque types are uninitialized, we should
allow their data length to be zero. However, most content
formatters seem to calculate the string length separately
so replace the pointer of empty data into a static string
that is guaranteed to be empty.

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