Fix/crashing when generating walls

[Azio-Pantheon]

Description

Added a nullptr check. It fixes #6279.

When slicing the file in #6279, there is a ACCESS_VIOLATION exception thrown by annotate_vertex(edge.vertex0(), edge.is_secondary() ? VertexCategory::OnContour : VertexCategory::Outside);. edge.vertex0() appears to be null in some cases. The root cause is still unknown, but adding a nullptr check to skip this line edge.vertex0() seems to be working.

Screenshots/Recordings/Graphs

Tests

OrcaSlicer can now slice the 3mf file user provided in the bug report.
Side.Entry.Housing.1.zip
I found these two lines in the debug log:

[warning]	2024-07-29
16:57:42.821800[Thread 0x00006a90]:Detected finite Voronoi vertex with non finite vertex, input polygons will be rotated back and forth.
[error]	2024-07-29 16:57:42.823801[Thread 0x00006a90]:Detected missing Voronoi vertex even after the rotation of input.

[Azio-Pantheon]

More info: I tried loading and slicing the .3mf file in PrusaSlicer and Bambu Studio.
PrusaSlicer can load and slice without any issue.
Bambu Studio fails to load the file(shows an empty plate). I then tried loading only the stl. which worked. Slicing the stl in Bambu Studio also crashes the app with no specific debug log. Interesting thing is, Bambu Studio crashes at a different stage:

[Noisyfox]

Prusa actually fixed this same issue here: prusa3d/PrusaSlicer@8784ca0

And I tested this patch on Orca and it does make that project sliced properly:

[Noisyfox]

Are you going to raise another PR for fixing this issue?

[Azio-Pantheon]

Are you going to raise another PR for fixing this issue?

Oh, I closed it because I thought we were getting the prusa fix. Glad I could help :D