How to deal gracefully with null foreign keys in Linq to SQL?

From c as Concession in Db.Concessions _
Select _
   c.ConcessionID, _
   c.Title, _
   c.Firm.Title

From c As Concession In Db.Concessions _
Join f As Firm In Db.Firms On f.FirmID Equals c.FirmID _
Select _
    c.ConcessionID, _
    c.Title, _
    Firm_Title = f.Title

from c in Db.Concessions
select
{
     c.ConcessionID,
     c.Title,
     Title = c.Firm.Title ?? ""
}

From c As Concession In db.Concessions _
Select _
    c.ConcessionID, _
    c.Title, _
    Firm_Title = If(c.Firm IsNot Nothing, c.Firm.Title, String.Empty) _

Public Property office() As String
            Get
                Return _office
            End Get
            Set(ByVal value As String)
                If value IsNot Nothing Then
                    _office = value
                Else
                    _office = String.Empty
                End If
            End Set
        End Property