fix: unable to view ticket if no assignee

[rtdany10]

Traceback (most recent call last):
  File "apps/frappe/frappe/app.py", line 95, in application
    response = frappe.api.handle()
  File "apps/frappe/frappe/api.py", line 54, in handle
    return frappe.handler.handle()
  File "apps/frappe/frappe/handler.py", line 47, in handle
    data = execute_cmd(cmd)
  File "apps/frappe/frappe/handler.py", line 85, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
  File "apps/frappe/frappe/__init__.py", line 1622, in call
    return fn(*args, **newargs)
  File "apps/helpdesk/helpdesk/helpdesk/doctype/hd_ticket/api.py", line 62, in get_one
    "assignee": get_assignee(ticket._assign),
  File "apps/helpdesk/helpdesk/helpdesk/doctype/hd_ticket/api.py", line 87, in get_assignee
    if len(j) < 1:
TypeError: object of type 'NoneType' has no len()
index-c9e099b3.js:269:1597

[codecov]

Codecov Report

Attention: 166 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
...hd_service_holiday_list/hd_service_holiday_list.py	36.92% <0.00%> (ø)
helpdesk/helpdesk/doctype/hd_action/hd_action.py	0.00% <0.00%> (ø)
...lpdesk/report/ticket_analytics/ticket_analytics.py	0.00% <0.00%> (ø)
...esk/doctype/hd_ticket_comment/hd_ticket_comment.py	0.00% <0.00%> (ø)
...k/doctype/hd_ticket_template/hd_ticket_template.py	0.00% <0.00%> (ø)
helpdesk/utils.py	0.00% <0.00%> (ø)
...elpdesk/helpdesk/doctype/hd_ticket_template/api.py	0.00% <0.00%> (ø)
helpdesk/helpdesk/doctype/hd_notification/utils.py	0.00% <0.00%> (ø)
...vice_level_agreement/hd_service_level_agreement.py	0.00% <0.00%> (ø)
helpdesk/helpdesk/doctype/hd_ticket/hd_ticket.py	0.00% <0.00%> (ø)
... and 2 more

📢 Thoughts on this report? Let us know!.

[rtdany10]

@ssiyad

[rtdany10]

@ssiyad