It is not immediately clear how these characters ended up on signs in
previous versions. It is clear, however, that they now render as empty
unicode boxes in 1.13, whereas previously they rendered as invisible
characters.
When these signs are loaded in versions after this commit, these
characters from the private use area of the Unicode block will be
stripped. The sign will then be marked to ensure this conversion only
runs once.
There is a flag -DPaper.keepInvalidUnicode=true that can be used if you
do not want us to strip these characters from your signs, though I can
think of no reason to use it.
Fixes GH-1571
