post update event creation failed when VNF update failed

Steps to Reproduce the issue
1. Create a vnfd with following data:
tosca_definitions_version: tosca_simple_profile_for_nfv_1_0_0

description: OpenWRT with services

metadata:
  template_name: OpenWRT

topology_template:
  node_templates:

    VDU1:
      type: tosca.nodes.nfv.VDU.Tacker
      capabilities:
        nfv_compute:
          properties:
            num_cpus: 1
            mem_size: 512 MB
            disk_size: 1 GB
      properties:
        image: OpenWRT
        config: |
          param0: key1
          param1: key2
        mgmt_driver: openwrt
        monitoring_policy:
          name: ping
          parameters:
            count: 3
            interval: 10
          actions:
            failure: respawn

    CP1:
      type: tosca.nodes.nfv.CP.Tacker
      properties:
        management: true
        order: 0
        anti_spoofing_protection: false
      requirements:
        - virtualLink:
            node: VL1
        - virtualBinding:
            node: VDU1

    CP2:
      type: tosca.nodes.nfv.CP.Tacker
      properties:
        order: 1
        anti_spoofing_protection: false
      requirements:
        - virtualLink:
            node: VL2
        - virtualBinding:
            node: VDU1

    CP3:
      type: tosca.nodes.nfv.CP.Tacker
      properties:
        order: 2
        anti_spoofing_protection: false
      requirements:
        - virtualLink:
            node: VL3
        - virtualBinding:
            node: VDU1

    VL1:
      type: tosca.nodes.nfv.VL
      properties:
        network_name: net_mgmt
        vendor: Tacker

    VL2:
      type: tosca.nodes.nfv.VL
      properties:
        network_name: net0
        vendor: Tacker

    VL3:
      type: tosca.nodes.nfv.VL
      properties:
        network_name: net1
        vendor: Tacker

2. Createa a vnf based on above vnfd.
3. Once vnf instance is up completely, do "sudo ifdown eth0" so that ssh will fail
4. Update vnf with any config file
5. vnf update fails and vnfm should create a post update event with an ERROR resource_state as expected but event create failed with a DB ERROR because the timestamp is None and is required.