Power Query patterns for changing default behavior when combining text values

powerquery-m-combiner-patterns.txt

//"a" & null = "a"
//null & "b" = "b"
//And this behavior which is different to the default behavior for Text.Combine:
//null & null = null
//Default separator is an empty string
custom_combine = (columns as list,optional sep as nullable text) => 
                        if 
                            ListAllNull(columns)
                        then 
                            null 
                        else 
                            Text.Combine(
                                columns,
                                if sep = null then "" else sep
                            ),