fix NPE for documents with nested table row groups

Vex creates an anonymous TableBox for nested row groups. Vex tried to
get pseudo elements for this anonymous boxes. This added a null key in
the StyleSheet cache and caused a NPE on flushing the style sheet.

Change-Id: I28f9f2bd69369ce0f579b87ad5f5d7a29c09feb5
Signed-off-by: Carsten Hiesserich <carsten.hie@gmail.com>
8 files changed