Fix a rare crash in the new LTN cell drawing

apps/ltn/src/connectivity.rs

@@ -262,6 +262,11 @@ fn setup_editing(
 
         // Highlight cell areas and their border areas when hovered
         for (idx, polygons) in render_cells.polygons_per_cell.iter().enumerate() {
+            // Edge case happening near https://www.openstreetmap.org/way/106879596
+            if polygons.is_empty() {
+                continue;
+            }
+
             let mut batch = GeomBatch::new();
             batch.extend(Color::YELLOW.alpha(0.1), polygons.clone());
             for arrow in neighbourhood.cells[idx].border_arrows(app) {

apps/ltn/src/draw_cells.rs

@@ -10,6 +10,7 @@ use crate::{colors, Neighbourhood};
 const RESOLUTION_M: f64 = 10.0;
 
 pub struct RenderCells {
+    /// Rarely, this might be empty if the area is very small
     pub polygons_per_cell: Vec<Vec<Polygon>>,
     /// Colors per cell, such that adjacent cells are colored differently
     pub colors: Vec<Color>,