[Fix] Add nil check for vault_data in show_handler

Prevent runtime errors when parsing Vault KV v2 responses if obj.data.data is nil.
This adds a safety check before accessing vault_data.selectors, consistent with
other handlers in the file (newkey_handler and roll_handler).

lualib/rspamadm/vault.lua

@@ -242,7 +242,10 @@ local function show_handler(opts, domain)
       -- For KV v2, data is nested under obj.data.data
       -- For KV v1, data is under obj.data
       local vault_data = opts.kv_version == 2 and obj.data.data or obj.data
-      return vault_data.selectors
+      if vault_data then
+        return vault_data.selectors
+      end
+      return nil
     end)
   end
 end