Table.SelectRows in conjunction with if statement from another table

Assuming that your REP and CSR are values and not tables, and you want to use one conditional statement, you would do it like this...

= Table.SelectRows(#"Sorted Rows", each if CSR = null
then [REP] = REP else [CSR] = CSR )

Example:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSlTSUUpRitWJVkoCslLhrDQwKxnISgezUuCsVCArQyk2FgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [CSR = _t, REP = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"CSR", type text}, {"REP", type text}}),
    CSR = null, //"b",
    REP = "h",
    SelectRows = Table.SelectRows(#"Changed Type" , each if CSR = null then [REP] = REP else [CSR] = CSR )
in
    SelectRows