[Bug] Add "Not assigned" to Assignee field on task page for tasks without an assigned user

[DrJohn1]

Display "Not assigned" on Assignee field in task page when not assigned to a user (bug)

Request Type

Bug

Work Environment

Question	Answer
OS version (server)	Ubuntu 18.04
OS version (client)	Ubuntu 18.04
TheHive version / git hash	4.0.0-1
Package Type	DEB

Problem Description

Tasks with no assigned users display an empty value for the Assignee field:

Steps to Reproduce

1. Create a new task but do not start it.
2. The assignee field has no text, hovering over makes the edit icon appear.

Possible Solutions

Add "Not assigned" as a placeholder to facilitate use of this field.