[processing]Fix "float division by zero" in polygonize algorithm if no
polygons were created.
For shapely 1.2.16+ and GEOS 3.2+ unary_union will be used for noding
lines - union method sometimes failed to create properly noded polyline.
Merge pull request #1149 from p0cisk/master

[processing]Fix "float division by zero" in polygonize algorithm if no