Meta elements are not recognised by Lua filters in topdown traversal

[kopeckyf]

There is a general strategy of skipping sub-trees in topdown traversal with Lua filters. For example, the following skips Cite objects:

traverse = 'topdown'

function Cite (n)
  return n, false
end

But it does not seem to work to filter for some objects.

In discussion #9369, @tarleb mentioned that this approach does not work for Meta elements because it is not implemented in the code and invited me to open an issue here.