The background image is visible even when the diagram contains element

[gcoutable]

Steps to reproduce

• Create a SysMLv2 project
• Create a general view, there is a background image
• Using the diagram palette, create a namespace import

Actual behavior

The namespace import is added to the diagram, but the background image is still visible

Expected behavior

The namespace import is added to the diagram, and the background image has been removed

[gcoutable]

The issue seems to occur because DiagramQueryElementService#isDiagramEmpty is too permissive