nixos/opendkim: Fix CapabilityBoundingSet option

An empty list results in no CapabilityBoundingSet at all, an empty
string however will set `CapabilityBoundingSet=`, which represents a
closed set.

Related: #120617

nixos/modules/services/mail/opendkim.nix

@@ -134,7 +134,7 @@ in {
         ReadWritePaths = [ cfg.keyPath ];
 
         AmbientCapabilities = [];
-        CapabilityBoundingSet = [];
+        CapabilityBoundingSet = "";
         DevicePolicy = "closed";
         LockPersonality = true;
         MemoryDenyWriteExecute = true;