Support d3.drag in testing environments

[Alasdair McLeay]

As per the report in #79 which was closed without a fix.

If using d3-drag in a non browser environment, such as jest + testing-library, an error will be thrown:

    Error: Uncaught [TypeError: Cannot read property 'document' of null]

This comes from

d3-drag/src/drag.js

Lines 50 to 63 in c6a7e46

 function mousedowned(event, d) { 
   if (touchending || !filter.call(this, event, d)) return; 
   var gesture = beforestart(this, container.call(this, event, d), event, d, "mouse"); 
   if (!gesture) return; 
   select(event.view) 
     .on("mousemove.drag", mousemoved, nonpassivecapture) 
     .on("mouseup.drag", mouseupped, nonpassivecapture); 
   nodrag(event.view); 
   nopropagation(event); 
   mousemoving = false; 
   mousedownx = event.clientX; 
   mousedowny = event.clientY; 
   gesture("start", event); 
 } 

d3-drag/src/nodrag.js

Lines 4 to 5 in c6a7e46

 export default function(view) { 
   var root = view.document.documentElement, 

Because view is null.

Downstream issues:

• xyflow/xyflow#2461
• xyflow/xyflow#2587

I think nodrag should return early if view is null:

export default function(view) {
  if(!view) {
    return;
  }
  var root = view.document.documentElement,
      selection = select(view).on("dragstart.drag", noevent, nonpassivecapture);
  if ("onselectstart" in root) {
    selection.on("selectstart.drag", noevent, nonpassivecapture);
  } else {
    root.__noselect = root.style.MozUserSelect;
    root.style.MozUserSelect = "none";
  }
}

...at least, this works for my use case. I'll raise a PR.