Fix api.contact.in_contact

The length of `links_in_contact` was always equal to number of links in the model

src/jaxsim/api/contact.py

@@ -100,7 +100,7 @@ def in_contact(
 
     link_names = link_names if link_names is not None else model.link_names()
 
-    if set(link_names) - set(model.link_names()) != set():
+    if (link_names := set(link_names)) and link_names.difference(model.link_names()):
         raise ValueError("One or more link names are not part of the model")
 
     from jaxsim.physics.algos.soft_contacts import collidable_points_pos_vel
@@ -124,7 +124,7 @@ def in_contact(
             below_terrain,
             jnp.zeros_like(below_terrain, dtype=bool),
         ).any()
-    )(jnp.arange(model.number_of_links()))
+    )(jnp.arange(len(link_names)))
 
     return links_in_contact