Breaking regression in v5.8.0

[rkirsling]

Although it was part of a minor release, #124 is a breaking change.

In particular, if we follow the docs and set fx and fy to null upon drag end (as in Force Dragging III), then we're in for some absurd behavior when we call simulation.nodes on our next data update.
(See Directed Graph Editor for a complete example.)

Setting them to undefined does work, since isNaN(undefined) is false while isNaN(null) is true.

[rkirsling]

Thanks a bunch for the quick fix!