[grid_map_costmap_2d] Costmap2DConverter - costTranslationTable for unknown cells

[naveedhd]

In Costmap2DConverter, addLayerFromCostmap2D uses costTranslationTable to convert values.

However this converts costmap_2d::NO_INFORMATION (255) into -1.0 which later if translated toOccupancyGrid, this value will be converted to 0.

The side effect of this would be that the unknown values to map will be translated as free space. costmap -> gridmap -> occupancy_grid 255 -> -1 -> 0

[naveedhd]

If the solution is to have additional check here (which makes value = -1 if it's less than zero), I can open a PR. or a better solution?