Order of inputs with multiple=True mixes when one of them becomes None

[janezd]

(Written by @markotoplak; moved from biolab/orange3#4215).

Describe the bug
When a user connects widgets to the multiple-input widget, these inputs appear in the widget in the order as they were connected. If then, one of the inputs becomes None, the receiving widget does not see them. If, then, it is set to something != None, the order changed.

To Reproduce

Make a workflow like this. Take care that you select things first in the lower Data Table, and then connect this output first to the Table on the right. The order of tabs is going to be (selection, the who DS).

Now, clear selection in the lower Table, and select something in it. The order of tabs in the right table changed.

I first discovered this with the Python Script widget, where this was more painful.

Expected behavior
The order should stay the same and the receiving widget should also see empty connection. Possible solution: "None"s are not filtered, but passed forward.

Orange version:
master