examples: Fix main default parameter

examples/notebook/examples/bus_driver_scheduling_sat.ipynb

@@ -2053,7 +2053,7 @@
     "        bus_driver_scheduling(False, num_drivers)\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    solve_bus_driver_scheduling()\n",
     "\n",
     "\n",

examples/notebook/examples/gate_scheduling_sat.ipynb

@@ -95,7 +95,7 @@
     "from ortools.sat.python import cp_model\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    \"\"\"Solves the gate scheduling problem.\"\"\"\n",
     "    model = cp_model.CpModel()\n",
     "\n",

examples/notebook/examples/golomb8.ipynb

@@ -160,7 +160,7 @@
     "    print(f'- total wall time: {solver.WallTime()}ms\\n')\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    solve_golomb_ruler(FLAGS.order)\n",
     "\n",
     "\n",

examples/notebook/examples/golomb_sat.ipynb

@@ -150,7 +150,7 @@
     "    print(f'- wall time: {solver.WallTime()}s\\n')\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    solve_golomb_ruler(FLAGS.order, FLAGS.params)\n",
     "\n",
     "\n",

examples/notebook/examples/hidato_sat.ipynb

@@ -257,7 +257,7 @@
     "            print('  - wall time : %f s' % solver.WallTime())\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    for pb in range(1, 7):\n",
     "        solve_hidato(build_puzzle(pb), pb)\n",
     "\n",

examples/notebook/examples/knapsack_2d_sat.ipynb

@@ -432,7 +432,7 @@
     "        solve_with_rotations(data, max_height, max_width)\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    solve_knapsack(FLAGS.model)\n",
     "\n",
     "\n",

examples/notebook/examples/linear_assignment_api.ipynb

@@ -117,7 +117,7 @@
     "            'Some input costs are too large and may cause an integer overflow.')\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    RunAssignmentOn4x4Matrix()\n",
     "\n",
     "\n",

examples/notebook/examples/pyflow_example.ipynb

@@ -140,7 +140,7 @@
     "        print('There was an issue with the min cost flow input.')\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    MaxFlow()\n",
     "    MinCostFlow()\n",
     "\n",

examples/notebook/examples/rcpsp_sat.ipynb

@@ -367,7 +367,7 @@
     "    solver.Solve(model)\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    rcpsp_parser = pywraprcpsp.RcpspParser()\n",
     "    rcpsp_parser.ParseFile(FLAGS.input)\n",
     "    SolveRcpsp(rcpsp_parser.Problem(), FLAGS.output_proto, FLAGS.params)\n",

examples/notebook/examples/shift_scheduling_sat.ipynb

@@ -483,7 +483,7 @@
     "    print('  - wall time       : %f s' % solver.WallTime())\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    solve_shift_scheduling(FLAGS.params, FLAGS.output_proto)\n",
     "\n",
     "\n",

examples/notebook/examples/steel_mill_slab_sat.ipynb

@@ -797,7 +797,7 @@
     "        print('No solution')\n",
     "\n",
     "\n",
-    "def main(_):\n",
+    "def main(_=None):\n",
     "    if FLAGS.solver == 'sat':\n",
     "        steel_mill_slab(FLAGS.problem, FLAGS.break_symmetries)\n",
     "    elif FLAGS.solver == 'sat_table':\n",