Bulk import to AD powershell

I'm trying to import en employee list into AD using powershell. The script needs to update exisisting AD objects.

The problem I have is that some users will have mobile number some internal numbers and some just an extension, when i run the Set-ADuser command in its most basic form it seems to fail whenever a value is blank.

I'm not great at scripting, can anyone explain to me why the script below doesnt work? And if so could anyone suggest how to make it work? Cheers

Import-module ActiveDirectory

$CSVPath = "c:\scripts\ictdepartment2.csv" 
$csvData = Import-CSV $CSVPath -delimiter "," -Header ("SamaccountName","identity","Title","Company","Department","Description","Office","OfficePhone","IPPhone","MobilePhone") 


##

foreach ($line in $csvData) 
{
$accountTable = @{ 
'SamaccountName'= $line.SamaccountName 
'identity'= $line.identity
'title'= $line.title 
'company'= $line.company 
'department'= $line.department 
'description'= $line.description 
'office'= $line.office
'officephone'= $line.officephone 
'IPPhone'= $line.ipPhone 
'mobilephone'= $line.mobilephone 
}
}

##

ForEach($line in $csvData) 
{
$SamaccountName = $line.SamaccountName
$identity = $line.identity
$title = $line.title 
$company = $line.company 
$department = $line.department 
$description = $line.description 
$office = $line.office
$officephone = $line.officephone
$IPPhone = $line.ipPhone 
$mobilephone = $line.mobilephone 
}

##

ForEach($line in $csvData) 
{
if ($officephone -ne $null)
{Set-ADUser -Identity $SamaccountName -OfficePhone $officephone}
if ($mobilephone -ne $null)
{Set-ADUser -Identity $SamaccountName -MobilePhone $mobilephone}
if ($ipphone -ne $null)
{Set-ADUser -Identity $SamaccountName -replace @{ipPhone=$ipphone}}

Set-ADUser -Identity $SamaccountName -Company $Company -Department $Department -Description $Description -Office $office -Title $title
}

If i use the following commands it works, however as soon as i add the if $null statement in it again fails :(

ForEach($line in $csvData) 
{ 
Get-ADUser -Filter "SamAccountName -eq '$($line.samaccountname)'" |
Set-ADUser -Replace @{ipphone=$ipphone}
}

ForEach($line in $csvData) 
{ 
Get-ADUser -Filter "SamAccountName -eq '$($line.samaccountname)'" |
Set-ADUser -Company $Company -Department $Department -Description $Description -MobilePhone $mobilephone -Office $office -OfficePhone $officephone -Title $title
}