Add check for nil property

Author: Rob Gries

libraries/provider_cisco_interface.rb

@@ -130,7 +130,8 @@ def set_layer3_combo_properties
       def set_layer2_properties
         # Disable switchport
         set_switchport_mode
-        @new_resource.switchport_trunk_allowed_vlan.delete!(' ')
+        @new_resource.switchport_trunk_allowed_vlan.delete!(' ') unless
+          @new_resource.switchport_trunk_allowed_vlan.nil?
         prop_set([:access_vlan, :switchport_autostate_exclude,
                   :switchport_trunk_allowed_vlan, :switchport_trunk_native_vlan,
                   :switchport_vtp