Fix 3D joint handling of collision exceptions, so that they lose effect when the joint is not valid in every case; also some redundant code removed.
Also avoid trying to create the joint when not neither body A nor body B are set.
Make 2D joints be handled like their 3D counterparts, which adds the fixes to them while also removing duplicated code.
Fixes#2383.
That year should bring the long-awaited OpenGL ES 3.0 compatible renderer
with state-of-the-art rendering techniques tuned to work as low as middle
end handheld devices - without compromising with the possibilities given
for higher end desktop games of course. Great times ahead for the Godot
community and the gamers that will play our games!
(cherry picked from commit c7bc44d5ad)
