Replace blank value

There are different possible kinds of blank values. Here's an example that starts with a text column containing an empty string, null, whitespace, and "null" and shows what the replace value function should look like in each case.

let
    Source = Table.FromColumns({{"", null, " ", "null"}}, {"Text"}),
    #"Replaced Value" = Table.ReplaceValue(Source,"","EmptyString",Replacer.ReplaceValue,{"Text"}),
    #"Replaced Value1" = Table.ReplaceValue(#"Replaced Value",null,"ActualNull",Replacer.ReplaceValue,{"Text"}),
    #"Replaced Value2" = Table.ReplaceValue(#"Replaced Value1"," ","Space",Replacer.ReplaceText,{"Text"}),
    #"Replaced Value3" = Table.ReplaceValue(#"Replaced Value2","null","TextNull",Replacer.ReplaceText,{"Text"})
in
    #"Replaced Value3"

Without seeing your specific table, I can't tell what sort of blank values you have. You can try adding a custom column to read the Unicode character(s) from your [TextColumn] like this and let me know what it returns:

Text.Combine(
    List.Transform(
        Text.ToList([TextColumn]),
        each Number.ToText(Character.ToNumber(_))
    ), ", ")

(An empty string will return an empty string, a null will throw an error, and any other character(s) should give a list of Unicode number(s).)