Fix dagre types to allow objects to be set as default edge labels (#19945)

The tests call `.setDefaultEdgeLabel(() => {})` which is a translation
to ES6 of something seen in the example in [Dagre's
readme](https://github.com/cpettitt/dagre/wiki#an-example-layout):
`g.setDefaultEdgeLabel(function() { return {}; });`. However, the
translation is botched: `() => {}` doesn't return an empty object, it
returns `undefined`, because `{}` is not an object literal, it's a
block with 0 statements.

I think this is what motivated the current typing to specify that `void`
can be returned by the callback. However, in my experience returning
`undefined` from this callback causes the library to crash. Thus I've
chagned the tests to say `.setDefaultEdgeLabel(() => ({}))` (i.e.
actually return an empty object, which is what the author probably
intended), and updated the types accordingly. This works in my project.

types/dagre/dagre-tests.ts

@@ -1,6 +1,6 @@
 const gDagre = new dagre.graphlib.Graph();
 gDagre.setGraph({})
-  .setDefaultEdgeLabel(() => {})
+  .setDefaultEdgeLabel(() => ({}))
   .setNode("a", {})
   .setEdge("b", "c")
   .setEdge("c", "d", {class: "class"});

types/dagre/index.d.ts

@@ -1,7 +1,10 @@
 // Type definitions for dagre 0.7
 // Project: https://github.com/cpettitt/dagre
-// Definitions by: Qinfeng Chen <https://github.com/qinfchen>, Lisa Vallfors <https://github.com/Frankrike>
+// Definitions by: Qinfeng Chen <https://github.com/qinfchen>
+//                 Lisa Vallfors <https://github.com/Frankrike>
+//                 Pete Vilter <https://github.com/vilterp>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 2.2
 
 export as namespace dagre;
 
@@ -11,8 +14,8 @@ export namespace graphlib {
         edge(id: any): any;
         nodes(): string[];
         node(id: any): any;
-        setDefaultEdgeLabel(callback: string|(() => string|void)): Graph;
-        setDefaultNodeLabel(callback: string|(() => string|void)): Graph;
+        setDefaultEdgeLabel(callback: string|(() => string|object)): Graph;
+        setDefaultNodeLabel(callback: string|(() => string|object)): Graph;
         setEdge(sourceId: string, targetId: string, options?: { [key: string]: any }, value?: string): Graph;
         setEdge(params: {v: string, w: string, name?: string}, value?: string): Graph;
         setGraph(label: GraphLabel): Graph;