nlesource: When standalone consider object.duration==0 as not set

nlesource: When standalone consider object.duration==0 as not set

nleobject.duration defaults to 0, but this is pretty unintuitive for
end user in the case nlesource is use standalone, just consider
duration=0 equivalent to duration=GST_CLOCK_TIME_NONE as it makes
the element much simpler to use, we could actually forbid 0 as a value
in the future.

Also take into account potential CLOCK_TIME_NONE