fix: fix plotting of null graphs with the Matplotlib backend

Author: ntamas

src/igraph/drawing/matplotlib/graph.py

@@ -532,7 +532,7 @@ def _draw_vertices(self):
 
         art = VertexCollection(
             patches,
-            offsets=offsets,
+            offsets=offsets if offsets else None,
             offset_transform=self.axes.transData,
             match_original=True,
             transform=Affine2D(),

tests/drawing/cairo/baseline_images/graph_null.png

@@ -85,6 +85,11 @@ def test_graph_with_curved_edges(self):
             backend="cairo",
         )
 
+    @image_comparison(baseline_images=["graph_null"])
+    def test_null_graph(self):
+        g = Graph()
+        plot(g, backend="cairo", target=result_image_folder / "graph_null.png")
+
 
 class ClusteringTestRunner(unittest.TestCase):
     @classmethod

tests/drawing/cairo/test_graph.py

@@ -155,6 +155,14 @@ def test_graph_with_curved_edges(self):
         )
         ax.set_aspect(1.0)
 
+    @image_comparison(baseline_images=["graph_null"])
+    def test_null_graph(self):
+        plt.close("all")
+        g = Graph()
+        fig, ax = plt.subplots()
+        plot(g, target=ax)
+        ax.set_aspect(1.0)
+
 
 class ClusteringTestRunner(unittest.TestCase):
     def setUp(self):