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