Fix linear approx in pinocchio ee kinematics

This commit fixes issue leggedrobotics#35

- CentroidalModelPinocchioMappingTpl::getOcs2Jacobian is called with Jv
  a 0×Nv matrix. This causes a runtime error in line 164 of pinocchio
  mapping.
- Subsequently
  PinocchioEndEffectorKinematics::getPositionLinearApproximation is
  broken when calling getOcs2Jacobian.
- By changing Jv to a zero matrix of size 3×Nv, the runtime error is
  fixed.

Author: zoenglinghou

ocs2_pinocchio/ocs2_pinocchio_interface/src/PinocchioEndEffectorKinematics.cpp

@@ -134,7 +134,7 @@ std::vector<VectorFunctionLinearApproximation> PinocchioEndEffectorKinematics::g
 
     VectorFunctionLinearApproximation pos;
     pos.f = data.oMf[frameId].translation();
-    std::tie(pos.dfdx, std::ignore) = mappingPtr_->getOcs2Jacobian(state, J.topRows<3>(), matrix_t::Zero(0, model.nv));
+    std::tie(pos.dfdx, std::ignore) = mappingPtr_->getOcs2Jacobian(state, J.topRows<3>(), matrix_t::Zero(3, model.nv));
     positions.emplace_back(std::move(pos));
   }
   return positions;