Add conversion test for empty graph

src/testing/graph_topology_conversion_testing.rs

@@ -10,6 +10,7 @@ macro_rules! graph_topology_conversion_tests {
     ($T:ty, $U:ty) => {
         $crate::graph_topology_conversion_tests!(@wrap $T, $U,
             convert,
+            convert_empty,
         );
     };
 
@@ -41,3 +42,22 @@ where
         |e| iso.edge(e),
     );
 }
+
+pub fn convert_empty<G: GraphTopologyConversion, H: MakeTestGraph>()
+where
+    G::Vertex: Debug,
+    G::Edge: Debug,
+    H::Vertex: Debug,
+    H::Edge: Debug,
+{
+    let source = H::empty();
+    let (graph, iso) = G::convert(&source);
+    fixtures::assert_isomorphic(
+        &source,
+        &graph,
+        &[],
+        |v| iso.vertex(v),
+        &[],
+        |e| iso.edge(e),
+    );
+}