importer logs an error when trying to create a custom_link to a Secret model which is removed in Nautobot

[lvrfrc87]

Environment

• Python version: 3.9.1
• Nautobot version: 1.0.2
• nautobot-netbox-importer version: 1.0.3

Expected Behavior

Imports to Secret should be skipped

Observed Behavior

importer return error when trying to create a custom_link to a Secret model which is removed in Nautobot

10:42:06 error    Nautobot reported a database integrity error
  action: create
  exception: null value in column "content_type_id" violates not-null constraint
DETAIL:  Failing row contains (f8f14a85-390f-59d1-937c-e8e0e94ddc47, 2021-06-23, 2021-06-23 10:42:06.834875+00, HP NA, HP-NA, {% if obj.primary_ip %}https://im-na.wdf.sap.corp/search.do?devi..., 100, CND, default, t, null).
  model: <class 'nautobot.extras.models.models.CustomLink'>
  model_data:
    {'button_class': 'default',
     'content_type': None,
     'created': None,
     'group_name': 'CND',
     'name': 'HP NA',
     'new_window': True,
     'pk': UUID('f8f14a85-390f-59d1-937c-e8e0e94ddc47'),
     'target_url': '{% if obj.primary_ip '
                   '%}https://im-na.wdf.sap.corp/search.do?device={{ '
                   'obj.primary_ip.address.ip '
                   '}}&pageTitle=Device+Search+Results&fo_device=contains&filterName=searchdevice&fo_managementStatus=equals&managementStatus=0&managementStatus=1&managementStatus=3&mergeMode=replace&basic=true&sortBy=hostName&filterType=Device&quickSearch=1{% '
                   'endif %}',
     'text': 'HP-NA',
     'weight': 100}

[glennmatthews]

Needs to be handled more generally in the case of CustomField/CustomLink/ExportTemplate/ObjectChange/etc. referencing a content-type that has no Nautobot equivalent. Probably a prerequisite for #58 and its sequels.