If table is empty, insert null value, else carry on.

Just create a table with 1 record, vs trying to add records to the empty table. See this:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i44FAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Test = _t]),
    Custom1 = 
        if Table.IsEmpty(Source) then 
            #table(
                {"Test"},
                    {
                        {1}
                    }
            ) else Source
in
    Custom1

The Source line has nothing it, just a Test column, no records, so Table.IsEmpty returns true.

So it generates this table:

If it isn't empty, then it returns the Source table.

You can change the {1} to {null} or {"Some text"} or whatever.

EDIT: I did look at your code more closely. It is hard to know why it is failing because you didn't show us what Changed Type3 was, and what it is dependent on. This code will add a null to the existing table though if it is empty:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i44FAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Test = _t]),
    #"AddNull" = if Table.IsEmpty(Source) then Table.InsertRows(Source,0, { [Test = "null"] }) else Source
in
    AddNull

I think your circular reference issue though is you are referring to other steps, and I cannot see what those steps are so they may have interdependencies that are causing it.