Make UserNote comment default to the empty string.

This make the behavior consistent between when UserNote doesn't exist and when comment is null.

The current behavior may return null in APIs, which misleads some clients doing feature detection into thinking the server does not support comments.
For example, see https://codeberg.org/husky/husky/issues/92

lib/pleroma/user_note.ex

@@ -31,7 +31,7 @@ defmodule Pleroma.UserNote do
            UserNote
            |> where(source_id: ^source.id, target_id: ^target.id)
            |> Repo.one() do
-      note.comment
+      note.comment || ""
     else
       _ -> ""
     end