fix(#3124): prevent empty icons_right_align response from breaking padding

[rossw01]

Bug is caused in builder.lua:

for _, d in ipairs(self.decorators) do
  add_to_end(rights, d:icons_right_align(not d:is(UserDecorator) and node or api_node))
end

In the case of tracked dotfiles, d:icons_right_align(not d:is(UserDecorator) and node or api_node) returns {} from the HiddenDecorator, but inside add_to_end(t1, t2) we only check if not t2. Not sure if this was intentional, I didn't notice any unexpected behaviour while testing but there may be edge cases I hadn't thought of.