[CP-SAT] do not add lb_tree_search when the model does not have an objective
[CP-SAT] do not add lb_tree_search when the model does not have an objective

or-tools/or-tools,or-tools/or-tools,google/or-tools,or-tools/or-tools,google/or-tools,or-tools/or-tools,google/or-tools,google/or-tools,google/or-tools,or-tools/or-tools,google/or-tools,or-tools/or-tools