Import Issues from CSV functionality doesn't allow the Assignee Field to be filled as unassigned

Problem Definition

The Import Issues from CSV functionality accessed through the search issues page (see image below) won't allow the assignee field of the CSV to be filled as unassigned.

For instance:
summary, issueType, assignee
summarytest, Bug, unassigned (won't work)
summarytest2, Bug, (will work)

Note: the CSV import functionality accessed through the admin page (see image below) allows it.

Suggested Solution

Allow the Import Issues from CSV functionality accessed through the search issues page to accept the assignee field as unassigned and reflect the behavior of the admin CSV import functionality.

Why this is important

Keep the consistency of functionalities behaviors. As we've checked that using the CSV importer from
(admin only) external system import this same behavior works.