Improve st_constraineddelaunay and st_delaunay and add new tests
Check null geometry
Fix st_constraineddelaunay.
If a geometrycollection contains only points or multipoints them a delaunay triangulation is applied.
Fix indentation and sentence
Merge pull request #344 from ebocher/st_constrainedDelaunayFix

St constrained delaunay 
About #336