Diagram are not well saved when contains an Actor shape

[CamiAndrei]

Steps to reproduce this issue:

• Insert a diagram m that doesn't exist yet in a page e.g.: Add in wiki mode: {{diagram reference="Diagram"/}} . "Diagram", doesn't exists.
• Click "Save and View"
• Click "Create diagram"
• Add some content in the diagram with shapes, an "Actor" shape

• Click Save&View Save&View it's grayed out and the page is not saved

  In the browser's console log the following error message is displayed:

  
  XML Parsing Error: unclosed token
  Location: http://.......:8080/xwiki/bin/edit/Sandbox/Diagramm?template=Diagram.DiagramTemplate
  Line Number 1, Column 1: Diagram:1:1
  Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://rt.draw.io/cache?alive. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).

TypeError: rectangles[(rectangles.length - 1)] is undefined DiagramEditSheet:604:27

![Screenshot from 2020-06-15 15-11-46](https://user-images.githubusercontent.com/12369283/84679506-0e3c0580-af3a-11ea-85dd-adb8dddc00c6.png)

 * Click Save. The Save button it's gray-out too.
 * Click Preview
 * Then Save&View from preview.
 * Go to the page where it was inserted 
 Only a link to the diagram is displayed. The diagram is not displayed.
 * Remove the "Actor" shape
  The diagram is displayed in the page where it was inserted.

![Screenshot from 2020-06-15 18-26-41](https://user-images.githubusercontent.com/12369283/84679969-a89c4900-af3a-11ea-81d9-63dc5f569c56.png)

 Also the diagram are not updated anymore in the pages where they were inserted, if at some point an Actor shape was added in it. 
* Add an Actor shape to an existing diagram 
* Click Save&View
 Save&View it's grayed out and the page is not saved
* Click Save. The Save button it's gray-out too.
 * Click Preview
 * Then Save&View from preview.
 * Go to the page where it was inserted 
The changes made in the diagram are not updated in the inserted page.

[oanalavinia]

This issue is duplicated to #144 , and as is explained in the comment https://github.com/xwikisas/application-diagram/issues/144#issuecomment-625782063 the issue is resumed to the problem that if the text is not inserted in a squared shape the diagram cannot be saved. I have updated the description to specify that it includes Actor shapes as well