Concatenate Multiple Rows And Switch Columns If Blank

I am using the following VBA to concatenate multiple results of Index Match formulas:

Function Lookup_concat(Search_string As String, _
  Search_in_col As Range, Return_val_col As Range)

Dim i As Long
Dim result As String

For i = 1 To Search_in_col.Count
  If Search_in_col.Cells(i, 1) = Search_string Then
    result = result & ", " & Return_val_col.Cells(i, 1).Value
  End If
Next

Lookup_concat = Mid(result, 3)

End Function

With the following formula:
=Lookup_concat(E2,Sheet2!$D$2:$D$2000,Sheet2!$N$2:$N$2000)

This is working well, however sometimes the N column in sheet2 contains a blank cell. In this scenario I want to return the value in the M column.


Please see attached xlsm.