How to convert DbNull to an empty string in Dapper

I am refactoring some code that used reflection to map a SqlCommand to a model. The new code uses the Dapper framework and works perfectly fine:

    Public Shared Function GetModel(Of T As New)(cmd As SqlCommand) As T
        Try

            Dim params = GetParametersDictionary(cmd)
            Dim result As Object

            Using conn As New SqlConnection(ConnectionString)
                result = conn.Query(Of T)(cmd.CommandText, params).FirstOrDefault()
            End Using

            Return result

        Catch ex As Exception
            ErrorsLogger.LogException(ex)
            Throw
        End Try
    End Function

However in the old code I had an edge case for string properties and DbNull values. Something like:

    If IsTypeOfString(prop) AndAlso IsDbNull(value) Then
        prop.SetValue(obj, "")
    End If

The new code does not convert DbNull (NULL) values to an empty string. Instead it leaves the value just not set aka Nothing, breaking some code elsewhere.

So my question is how to change this function to convert NULL values to an empty string? Preferably without reflection.

Things tried / considered / constraints:

• Custom Dapper SqlMapper: Does not work when values are NULL.
• Custom model mapper: Cannot create a custom model mapper for every model type.
• Init strings as "" in class constructor: Cannot guaranty that every string property is correctly initialized with "".
• Cannot change the queries or model classes (at least for now).