object: minor optimizations
object: minor optimizations

	* src/object/object.cc: Use empty() instead of checking the size.
	Rely on delete 0 being valid.
	Factor.
	Inline short auxiliary function.

aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi