fix: handle nil SNMP community string in discovery logging

- Check if community string is nil before slicing
- Display '(none)' instead of crashing with FunctionClauseError
- Fixes discovery crashes when community string is not set

Error was: String.slice(nil, 0..3) causing discovery tasks to fail

lib/towerops/snmp/deferred_discovery.ex

@@ -181,12 +181,18 @@ defmodule Towerops.Snmp.DeferredDiscovery do
         port = Keyword.get(client_opts, :port, 161)
         adapter = Keyword.get(client_opts, :adapter, "Real SNMP")
 
+        # Safely format community string (may be nil)
+        community_display =
+          if community && String.length(community) > 0,
+            do: String.slice(community, 0..3) <> "***",
+            else: "(none)"
+
         Logger.warning(
           "Device #{ip} categorized as unresponsive - SNMP GET failed",
           ip: ip,
           port: port,
           version: version,
-          community: String.slice(community, 0..3) <> "***",
+          community: community_display,
           adapter: inspect(adapter),
           error: inspect(reason)
         )