Fix mesh errors in SeparationRayShape3D debug fills

[BattyBovine]

Fixes an issue where SeparationRayShape3D would generate errors when debug meshes are enabled, due to the ray's debug mesh being generated as an empty array. This fix adds a single triangle to the debug mesh, which follows the shape of the debug line.

Closes #100665

Edit: This fixes the same issue as #101014, but using a less than ideal workaround. I will leave this pull request open for now, but the other fix is likely to be preferable.

[smix8]

Should prevent the CollisionShape3DGizmoPlugin from adding empty surfaceless meshes. That would avoid the need to add dummy meshes and solve that issues for any kind of other shape.

The code where the gizmo adds the fill mesh can be found in the CollisionShape3DGizmoPlugin::redraw() function. E.g. extend the mesh check from just check if it is valid to checking its surface count and reject if surface count is zero.

[BattyBovine]

I decided to take your advice and made another pull request (#101014) that adds the necessary checks for an empty mesh. If that fix is preferred, we can merge that instead. There should be no issue merging both if desired, but an empty surface check is much more resistant to similar issues in the future, and prevents the need for this very specific fix.