nixos/opendkim: Fix CapabilityBoundingSet option
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/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs