Bug 735303 - Don't create empty groups from VDX

Fixed in the VDX plug-in itself but also on the lib API level
with g_return_val_if_fail (objects != NULL, NULL);

(cherry picked from commit ec4dbbcbc21e1233e46ba9ae059a18bed459ca24)
Dia 0.97.3 - bug-fix release (NEWS)

dia-0.97.3: 2014-09-05

 * Fix double free with some SVG rendering (regression from Dia 0.97.2)
 * Fixes to cope better with updated versions of Dia's dependencies:
   - don't crash at start-up with ABI breaking GLib 2-36
   - don't assert in cairo 1.12 with invalid arc parameters
   - avoid kerning problems (character overlap) for all Pango versions
   - fix image files to be loadable by libpng16
 * Backport fixes for some seldom crashes and other annoyances, see:
   http://bugzilla.gnome.org/buglist.cgi?product=dia&target_milestone=0.97.3
 * Update translations for Brazilian Portuguese, German, Hungarian,
   Polish, Romanian, Serbian and Slovenian