core: fix potential exception case in SSHExec middleware

lib/vagrant/action/builtin/ssh_exec.rb

@@ -28,12 +28,12 @@ module Vagrant
           # not yet ready for SSH, so we raise this exception.
           raise Errors::SSHNotReady if info.nil?
 
-          if info[:private_key_path]
+          info[:private_key_path] ||= []
+
           # Check SSH key permissions
           info[:private_key_path].each do |path|
             SSH.check_key_permissions(Pathname.new(path))
           end
-          end
 
           if info[:private_key_path].empty? && info[:password]
             env[:ui].warn(I18n.t("vagrant.ssh_exec_password"))