Skip loading linked models in the drawing if they're unloaded #6403

As it's probably unintuitive (user can't see the linked model in viewport but we still load it) and user then have no option to keep model in Links but keep it unloaded.

Also producing traceback below if linked file is missing.
```
Error: Python: Traceback (most recent call last):
  File "\bonsai\bim\module\drawing\operator.py", line 299, in execute
    linework_svg = self.generate_linework(context)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "\bonsai\bim\module\drawing\operator.py", line 828, in generate_linework
    IfcStore.session_files[link.name] = ifcopenshell.open(link.name)
                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "\ifcopenshell\__init__.py", line 162, in open
    raise FileNotFoundError(f"File does not exist: '{path}'.")
FileNotFoundError: File does not exist: 'D:\Dropbox\GitLab\OD_Library\BIM\OD_Revit_Template - 2020 - IFC4 - Reference View.ifc'.
```

src/bonsai/bonsai/bim/module/drawing/operator.py

@@ -824,6 +824,8 @@ class CreateDrawing(bpy.types.Operator):
 
         props = tool.Project.get_project_props()
         for link in props.links:
+            if not link.is_loaded:
+                continue
             if link.name not in IfcStore.session_files:
                 IfcStore.session_files[link.name] = ifcopenshell.open(link.name)
             files[link.name] = IfcStore.session_files[link.name]