What is in these cells?

A fringe case perhaps, but your function would return TRUE if the cell contains only a prefix character. I think a generic UDF (without error handling) would be:

Public Function HasNullString(ByVal rngCell As Range) As Boolean
    
    Dim rngFirstCell As Range
    
    Set rngFirstCell = rngCell.Cells(1)
    
    If Not VBA.IsEmpty(rngFirstCell) Then
        If rngFirstCell.Formula = vbNullString Then
            HasNullString = (VBA.LenB(rngCell.PrefixCharacter) = 0)
        End If
    End If
    
End Function

I haven't properly tested it, so I might have missed something.

I've never needed to use a function like this, so I think any problems with this would be fairly unusual.