allow emtpy topology to be served

common/client-core/src/client/topology_control/nym_api_provider.rs

@@ -102,14 +102,18 @@ impl NymApiTopologyProvider {
 
         let topology = nym_topology_from_detailed(mixnodes, gateways)
             .with_epoch(current_epoch)
-            .with_all_mixes(all_mixes)
-            .with_all_gateways(all_gateways)
+            .with_all_mixes(all_mixes.clone())
+            .with_all_gateways(all_gateways.clone())
             .filter_system_version(&self.client_version);
 
         if let Err(err) = self.check_layer_distribution(&topology) {
             warn!("The current filtered active topology has extremely skewed layer distribution. It cannot be used: {err}");
             self.use_next_nym_api();
-            None
+            let empty_topology = NymTopology::empty()
+                .with_epoch(current_epoch)
+                .with_all_mixes(all_mixes)
+                .with_all_gateways(all_gateways);
+            Some(empty_topology)
         } else {
             Some(topology)
         }

common/topology/src/lib.rs

@@ -88,6 +88,16 @@ impl NymTopology {
         }
     }
 
+    pub fn empty() -> Self {
+        NymTopology {
+            mixes: BTreeMap::new(),
+            gateways: Vec::new(),
+            epoch: 0,
+            all_mixes: Vec::new(),
+            all_gateways: Vec::new(),
+        }
+    }
+
     pub fn with_epoch(mut self, epoch: EpochId) -> Self {
         self.epoch = epoch;
         self