Network: Handle null data gracefully (#3571)

* Network: preload images in options for all shape types

Fixes #3532

If the images option is set, preload it, even if the node shape is not `image` or `circularImage`.
This needs to be done because the user can switch to an image shape later, as is happening in the issue.
Option `image` is only mandatory for the image shapes.

There is no unit test for this, because it would need a mock object for Image and I didn't succeed in creating/finding one.

* Network: Handle null data gracefully

During testing I discovered that passing `null` for network data leads to a thrown error.
This adds a guard to prevent it, plus unit tests for regression.

lib/network/Network.js

@@ -154,6 +154,10 @@ Emitter(Network.prototype);
  * @param {Object} options
  */
 Network.prototype.setOptions = function (options) {
+  if (options === null) {
+    options = undefined;  // This ensures that options handling doesn't crash in the handling
+  }
+
   if (options !== undefined) {
     let errorFound = Validator.validate(options, allOptions);
     if (errorFound === true) {

test/Network.test.js

@@ -395,6 +395,25 @@ describe('Network', function () {
     createSampleNetwork(options);
   });
 
+
+  it('can deal with null data', function() {
+    // While we're at it, try out other silly values as well
+    // All the following are wrong, but none should lead to a crash
+    var awkwardData = [
+      null,
+      [1,2,3],
+      42,
+      'meow'
+    ];
+
+    var container = document.getElementById('mynetwork');
+
+    for (var n = 0; n < awkwardData.length; ++n) {
+      var network = new vis.Network(container, awkwardData[n], {});  // Should not throw
+    }
+  });
+
+
 describe('Node', function () {
 
   it('has known font options', function () {