Replace Blank or Nulls based on Column Type

Create a list of Transforms depending on the column types, then apply that.

For example, if you wanted to apply your above transformations to every column in a table of those data types, you could use:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Ncw7EoAgFEPRvaSmUfGzlzevEBEsGDr2b8yM1T1pYoYTARM8GIhZSNQiXZTQR2u08u3MEaWbWqVCbf+TWoldeqgD7i8=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t]),
    #"Replaced Value" = Table.ReplaceValue(Source,"null",null,Replacer.ReplaceValue,{"Column1"}),
    #"Previous Step" = Table.TransformColumnTypes(#"Replaced Value",{{"Column1", type text}, {"Column2", Int64.Type}}),

//Use this part below in your code
//  changing #"Previous Step" to whatever your previous step is
xFormList = 
    List.Transform(
        Table.SelectRows(Table.Schema(#"Previous Step"), 
            each [TypeName]="Text.Type")[Name], (L)=>{L, each if _ = "" or _ = null then "NA" else _, type text}) & 
    List.Transform(
        Table.SelectRows(Table.Schema(#"Previous Step"), 
            each [TypeName]="Int64.Type")[Name],(L)=>{L, each if _ = "" or _ = null then 0 else _, Int64.Type}),

    replaceM = Table.TransformColumns(#"Previous Step", xFormList)
in
    replaceM