planner_cspace: fix partial costmap update with unknown cells

[at-wat]

related to #153

[at-wat]

PASSED

All tests passed

build/test_results/costmap_cspace/gtest-test_costmap_3d.xml: 6 tests
build/test_results/costmap_cspace/roslint-costmap_cspace.xml: 1 tests
build/test_results/map_organizer/roslint-map_organizer.xml: 1 tests
build/test_results/obj_to_pointcloud/roslint-obj_to_pointcloud.xml: 1 tests
build/test_results/planner_cspace/gtest-test_blockmem_gridmap.xml: 2 tests
build/test_results/planner_cspace/gtest-test_cyclic_vec.xml: 7 tests
build/test_results/planner_cspace/roslint-planner_cspace.xml: 1 tests
build/test_results/planner_cspace/rostest-test_test_abort_rostest.xml: 1 tests
build/test_results/planner_cspace/rostest-test_test_navigation_rostest.xml: 1 tests
build/test_results/planner_cspace/rostest-test_test_preempt_rostest.xml: 1 tests
build/test_results/planner_cspace/rosunit-test_abort.xml: 1 tests
build/test_results/planner_cspace/rosunit-test_navigate.xml: 1 tests
build/test_results/planner_cspace/rosunit-test_preempt.xml: 1 tests
build/test_results/safety_limiter/roslint-safety_limiter.xml: 1 tests
build/test_results/safety_limiter/rostest-test_test_safety_limiter_rostest.xml: 1 tests
build/test_results/safety_limiter/rosunit-test_safety_limiter.xml: 1 tests
build/test_results/track_odometry/gtest-tf_projection_test.xml: 1 tests
build/test_results/track_odometry/roslint-track_odometry.xml: 1 tests
build/test_results/trajectory_tracker/roslint-trajectory_tracker.xml: 1 tests
Summary: 31 tests, 0 errors, 0 failures, 0 skipped