(One difference is that for an XFListLevelBullet for which SetBulletChar has not
been called, ToXml will use an empty string now instead of a string containing a
single NUL character.  But given that initializing m_chBullet to zero in the
XFListLevelBullet ctor was only added recently with
72069675 "coverity#738770: Unitialized scalar
field," assume that ToXml is supposed to be only called after SetBulletChar has
been called.)

Change-Id: I7a5d7496b038cd094ab11ef8cbc1823ffc5bc130