AbstractFontPart tmpFontDir will remain null if no user.home

[amundsor]

AbstractFontPart#63 warns the user about this:
log.warn("No home dir found; consider setting property 'docx4j.openpackaging.parts.WordprocessingML.ObfuscatedFontPart.tmpFontDir'");

But when trying to convert Word -> PDF when user.home is not present, will create NullPointerException in
ObfuscatedFontPart#227.

Perhaps this is an acceptable behaviour, but should the log-message be altered or throwing exception?