People search relevance is not optimized when the Active Directory has errors in the manager reporting structure

I often see this warning reported by SharePoint Health Analyzer:

People search relevance is not optimized when the Active Directory has errors in the manager reporting structure

There are lots of users in our organisation so I use the Get-ADUser PowerShell cmdlet to identify users without the manager property set, and output to a CSV file:

Import-Module ActiveDirectory
Get-ADUser -Filter * | Where-Object {$_.Manager -Eq $Null} | Export-CSV C:\list.csv

Now I can easily populate the Manager property for those users in the list.