* 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.