Replace Null Values and If

Centaur Based on your pattern it looks like you only need the last alphabet/word in that string so you don't need to use conditional logic here instead you can extract the last charcater with Text.End if it is an alphabet and if it is a word then you can split it with Text.Split and take the last entry in the list.

let
    Source = Table.FromRows (
        Json.Document (
            Binary.Decompress (
                Binary.FromText (
                    "i45Wcs5XcFTSUYoAAqVYHTDfCciHsZ2B7EgQgAm4gCVjAQ==",
                    BinaryEncoding.Base64
                ),
                Compression.Deflate
            )
        ),
        let
            _t = ((type nullable text) meta [Serialized.Text = true])
        in
            type table [ Vendor = _t, #"Budget Category" = _t ]
    ),
    ReplacedValue = Table.ReplaceValue (
        Source,
        "",
        null,
        Replacer.ReplaceValue,
        { "Budget Category" }
    ),
    If_Alphabet = Table.AddColumn (
        ReplacedValue,
        "Alphabet",
        each 
            if [Budget Category] is null 
            then Text.End ( [Vendor], 1 ) 
            else [Budget Category],
        type text
    ),
    If_Word = Table.AddColumn (
        If_Alphabet,
        "Word",
        each
            if [Budget Category] is null 
            then List.Last ( Text.Split ( [Vendor], " " ) )
            else [Budget Category],
        type text
    )
in
    If_Word