Code wont work if the first cells are empty

Hello there,

I have this code and it doesn't work when for instance its starts at row 6 but cells 6,7,8,9 are empty the code won't work and won't cycle through the rest of them.

Is there a way to fix this?

Private Sub Worksheet_Activate()
Dim rowNumber As Long
Dim thisSheet As Worksheet
Dim cellToCheck As Range
Dim LLName As String
Dim LFName As String
Dim LTitle As String
Dim LResponse As Integer
Set thisSheet = ActiveSheet
rowNumber = 6
    Do Until thisSheet.Cells(rowNumber, 14).Value = vbNullString
        Set cellToCheck = thisSheet.Cells(rowNumber, 14)
            If cellToCheck.Interior.ColorIndex = xlColorIndexNone Then
            If cellToCheck.Value = DateAdd("d", -1, Date) Then
        LTitle = Sheets("Sheet1").Range("D" & rowNumber).Value
        LFName = Sheets("Sheet1").Range("E" & rowNumber).Value
        LLName = Sheets("Sheet1").Range("F" & rowNumber).Value
            LResponse = MsgBox("Did " & LTitle & " " & LFName & " " & LLName & " pass their course yesterday?", vbYesNoCancel, "Course")
                            If LResponse = vbYes Then
                cellToCheck.Interior.Color = RGB(0, 255, 0)
           End If
           If LResponse = vbNo Then
                cellToCheck.Interior.Color = RGB(255, 0, 0)
                End If
            If LResponse = vbCancel Then
            Exit Sub
            End If
        End If
    End If
rowNumber = rowNumber + 1
Loop
End Sub

KJD