cleanup: PagerankGraph getters return undefined

PagerankGraph's `node` and `edge` getters returned null for unavailable
entries, rather than undefined. This is inconsistent with general JS
behavior, and with the base Graph. I've now cleaned it up.

Test plan: unit tests updated; `yarn test` passes.

src/core/pagerankGraph.js

@@ -296,7 +296,7 @@ export class PagerankGraph {
     this._verifyGraphNotModified();
     const score = this._scores.get(x);
     if (score == null) {
-      return null;
+      return undefined;
     } else {
       return {address: x, score};
     }
@@ -334,7 +334,7 @@ export class PagerankGraph {
       const weight = NullUtil.get(this._edgeWeights.get(edge.address));
       return {edge, weight};
     }
-    return null;
+    return undefined;
   }
 
   /**

src/core/pagerankGraph.test.js

@@ -80,10 +80,10 @@ describe("core/pagerankGraph", () => {
   });
 
   describe("node / nodes", () => {
-    it("node returns null for node not in the graph", () => {
+    it("node returns undefined for node not in the graph", () => {
       const g = nonEmptyGraph();
       const pg = new PagerankGraph(g, defaultEvaluator);
-      expect(pg.node(NodeAddress.empty)).toEqual(null);
+      expect(pg.node(NodeAddress.empty)).toBe(undefined);
     });
     it("nodes yields the same nodes as are in the graph", () => {
       const g = advancedGraph().graph1();
@@ -189,9 +189,9 @@ describe("core/pagerankGraph", () => {
       }
     });
 
-    it("edge returns null for address not in the graph", () => {
+    it("edge returns undefined for address not in the graph", () => {
       const pg = new PagerankGraph(nonEmptyGraph(), defaultEvaluator);
-      expect(pg.edge(EdgeAddress.empty)).toEqual(null);
+      expect(pg.edge(EdgeAddress.empty)).toBe(undefined);
     });
 
     it("edge and edges both throw an error if underlying graph is modified", () => {