Ho ! This is simple : it's an ugly hack
With the provious code, it was hard to attach an element (with a joint) to the "unmovable world", since this world is in facts the objet "GLOBAL" with its element "ground". But ground is a satic element (RAYDIUM_ODE_STATIC) and it's not possible to attach a joint to a static element.
So I added a faked (out-of-bounds) element with a NULL body, since NULL means "static world" for an ODE joint (see official doc), identified be the RAYDIUM_ODE_JOINT_FIXED constant.
There will be an example of such use in the shader demo (I've added this feature since I was needing it for this demo).