Custom definitions fail if parent definition has no properties

[Валерия Валерьевна Швецова]

I have a problem with custom definitions when a parent definition doesn't have a properties field.
Custom definition file openstack-flavor-definition.yaml:

------------------------------------------------------------
tosca_definitions_version: tosca_simple_yaml_1_0

capability_types:
  openstack.capabilities.OpenstackRoot:
    derived_from: tosca.capabilities.Root

  openstack.capabilities.Flavor:
    derived_from: openstack.capabilities.OpenstackRoot
    properties:
      disk:
        type: scalar-unit.size
        required: false
      ephemeral:
        type: scalar-unit.size
        default: 0 GiB
      id:
        type: string
        required: false
      is_public:
        type: boolean
        default: true
      name:
        type: string
        required: true
      ram:
        type: scalar-unit.size
        required: false
      swap:
        type: scalar-unit.size
        default: 0 MiB
      vcpus:
        type: integer
        required: false

node_types:
  openstack.nodes.Flavor:
    derived_from: tosca.nodes.Root
    properties:
      extra_specs:
        type: map
        required: false
    capabilities:
      self:
        type: openstack.capabilities.Flavor
        occurrences: [ 1, 1 ]
------------------------------------------------------------

Template file openstack-flavor-template.yaml:
------------------------------------------------------------
tosca_definitions_version: tosca_simple_yaml_1_0

imports:
  - openstack-flavor-definition.yaml

topology_template:
  node_templates:
    m1.small:
      type: openstack.nodes.Flavor
      capabilities:
        self:
          properties:
            disk: 150 GiB
            name: m1.small
            ram: 2 GiB
            vcpus: 1
------------------------------------------------------------

The command:
tosca-parser --template openstack-flavor-template.yaml

Above command gets into error:
------------------------------------------------------------
Traceback (most recent call last):
  File "/home/user/virt-env/tosca-parser/bin/tosca-parser", line 10, in <module>
    sys.exit(main())
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/shell.py", line 116, in main
    ParserShell().main(args)
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/shell.py", line 59, in main
    self.parse(path)
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/shell.py", line 68, in parse
    tosca = ToscaTemplate(path, None, a_file)
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/tosca_template.py", line 101, in __init__
    self.topology_template = self._topology_template()
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/tosca_template.py", line 120, in _topology_template
    None)
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/topology_template.py", line 57, in __init__
    self.nodetemplates = self._nodetemplates()
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/topology_template.py", line 97, in _nodetemplates
    tpl.validate(self)
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/nodetemplate.py", line 188, in validate
    self._validate_capabilities()
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/entity_template.py", line 184, in _validate_capabilities
    self._validate_capabilities_properties(capabilities)
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/entity_template.py", line 193, in _validate_capabilities_properties
    props[self.PROPERTIES])
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/entity_template.py", line 218, in _common_validate_properties
    for p in entitytype.get_properties_def_objects():
  File "/home/user/virt-env/tosca-parser/local/lib/python2.7/site-packages/toscaparser/elements/capabilitytype.py", line 44, in get_properties_def_objects
    for prop, schema in props.items():
AttributeError: 'NoneType' object has no attribute 'items'
------------------------------------------------------------

Expected output:

version: tosca_simple_yaml_1_0

nodetemplates:
 m1.small