handle $NONE$ net features

[nam20485]

Spec pg. 116:

When a feature does not have a net defined it is assigned to NET $NONE$. All un-assigned outer layer pads are defined as NET $NONE$. With more than one $NONE$ net, each is disconnected from the other. Any two points of a $NONE$ net can be connected or disconnected, depending on the design.

Features in the layer/features file with no corresponding FIDs under the eda/data NET/SNT records?