Replace null values in multipe columns with a different column

Hello - this is one way you can do it.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WckrNU9JRci3KTAZSUBSrE63kVpSagpCAqILJeWXm5laiS0I1gBUEJ+eXlGAaGwsA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Name = _t, L1 = _t, L2 = _t, L3 = _t, L4 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Name", type text}, {"L1", type text}, {"L2", type text}, {"L3", type text}, {"L4", type text}}),
    #"Replaced Value" = Table.ReplaceValue(#"Changed Type","",null,Replacer.ReplaceValue,{"L2", "L3", "L4"}),
    Custom3 = Table.AddColumn ( #"Replaced Value", "FieldValues", each Record.FieldValues ( _ ) ),
    ReplFirstNull = Table.AddColumn ( 
        Custom3, 
        "New", each 
        let 
            varFirstNonNullPosition = List.NonNullCount ( [FieldValues] ),
            ReplaceFirstNull = Text.Combine ( List.ReplaceRange ( [FieldValues], varFirstNonNullPosition, 1, { [FieldValues]{0} } ), "|" )
        in
            ReplaceFirstNull
    ),
    #"Removed Other Columns" = Table.SelectColumns(ReplFirstNull,{"New"}),
    #"Split Column by Delimiter" = Table.SplitColumn ( #"Removed Other Columns", "New", Splitter.SplitTextByDelimiter("|", QuoteStyle.Csv), Table.ColumnNames ( Source ) )
in
    #"Split Column by Delimiter"