Only return a writeguard for routes if there are routes

Return None in case the entry is queried or not existing, as we
semantically don't have routes in this case. This matches the
implementatin of routes_mut on the WriteGuard

Signed-off-by: Lee Smet <lee.smet@hotmail.com>

mycelium/src/routing_table.rs

@@ -201,16 +201,20 @@ impl RoutingTable {
             .exact_match(subnet_address, subnet.prefix_len().into())?
             .clone();
 
-        Some(WriteGuard {
-            routing_table: self,
-            // If we didn't find a route list in the route table we create a new empty list,
-            // therefore we immediately own it.
-            value,
-            exists: true,
-            subnet,
-            expired_route_entry_sink: self.expired_route_entry_sink.clone(),
-            cancellation_token: self.cancel_token.clone(),
-        })
+        if matches!(*value, SubnetEntry::Exists { .. }) {
+            Some(WriteGuard {
+                routing_table: self,
+                // If we didn't find a route list in the route table we create a new empty list,
+                // therefore we immediately own it.
+                value,
+                exists: true,
+                subnet,
+                expired_route_entry_sink: self.expired_route_entry_sink.clone(),
+                cancellation_token: self.cancel_token.clone(),
+            })
+        } else {
+            None
+        }
     }
 
     /// Adds a new [`Subnet`] to the `RoutingTable`. The returned [`WriteGuard`] can be used to