Fix #4688: Malformed SQL when searching -status:any.

Negating `Post.all` produced the invalid SQL fragment `WHERE NOT ()`.
Use `Post.where("TRUE")` instead to produce `WHERE NOT (TRUE)`.

app/logical/post_query_builder.rb

@@ -290,7 +290,7 @@ class PostQueryBuilder
     when "unmoderated"
       Post.in_modqueue.available_for_moderation(current_user, hidden: false)
     when "all", "any"
-      Post.all
+      Post.where("TRUE")
     else
       Post.none
     end