Do not error on missing ZWNJ (ligature break) characters.

Do not error on missing ZWNJ (ligature break) characters.

Since TeXLive 2016, "fontspec" maps the ligature break command
\textcompwordmark to the ZWNJ character (U+200C).
This character is missing in many fonts (including the default: Latin
Modern) which leads to "Missing character" warnings in the XeTeX/LuaTeX
log file if a document using non-TeX fonts contains a ligature break.

LyX reports missing characters as error since fixing #9610.
In case of "invisible" characters, there is no data loss, in case of the
ZWNJ the functionality is kept: ligatures are prevented also if the ZWNJ
is missing in a font.

Therefore, a missing ZWNJ is now treated similar to missing characters
in "nullfont" (see [63f41711/lyxgit], bug #10394) and does not trigger
an error.

Fixes: #10727
(cherry picked from commit a40868510d36ccdcf3c431cdee17f8204d4b258a)