Rendering graph with initial node positions doesn't properly place nodes without links

[TranquilMarmot]

Describe the bug
When rendering the graph and giving nodes initial positions via x/y, if a node does not have any links it will not be properly placed in the graph.

To Reproduce

Here's a minimal repo:

import React, { Component } from "react";
import { Graph } from "react-d3-graph";
import "./App.css";

class App extends Component {
  render() {
    const data = {
      nodes: [
        {
          id: "node-1",
          fx: 100,
          fy: 100
        },
        {
          id: "node-2",
          fx: 200,
          fy: 200
        },
        {
          id: "node-3",
          fx: 300,
          fy: 300
        }
      ],
      links: [
        {
          source: "node-1",
          target: "node-2"
        }
      ]
    };

    const config = {
      width: 2000,
      height: 2000
    };

    return (
      <div className="App">
        <Graph id="my-graph" data={data} config={config} />
      </div>
    );
  }
}

export default App;

I also have this in a code sandbox: https://codesandbox.io/s/create-react-app-7ozk6?file=/src/App.js

You'll notice that node-1 and node-2 are placed properly, but node-3 is placed at 0,0.

Adding i.e. { source: "node-2", target: "node-3" } to the links array will cause node-3 to be properly placed.

Expected behavior
Nodes that have x/y coordinates should always be placed in the right spot when rendering even if they don't have links.

Screenshots

Without links:

With links:

Environment:

• OS: Windows, Ubuntu
• Browser: Chrome, Firefox
• Version: Chrome 78, Firefox 79
• Node version: v12.13.0
• react-d3-graph version: 2.5.0
• d3 version: Whatever comes along with react-d3-graph
• react version: 16.13.1