[URDF][C++][Bug] Merge link with parent only if it has an inertial tag.

  * Inertial-less links under fixed joints exist for semantic
    purposes, i.e. to provide referential frames.
  * Keep storing these links as fixed bodies.
  * The case of pairs with non-fixed joints and inertial-less links
    still needs to be properly addresses, as the current behavior
    (identity inertia) is clearly bad.