Map free_thresh default value may cuase problems.

[AmmarAlbakri]

This is not a bug per se, because it can be fixed easily,

Nonetheless, if you are facing this problem, here is a rundown of the reason and the solution:

If your map is showing like this:

When you are expecting it to show like this:

Then this is likely due to the value free_thresh in the map .yaml file being set to 0.25, setting the values to something like 0.15 solves the problem and makes a clear distinction between free and unknown cells.

In ROS 1 this value was set to a default of 0.196, dunno why the default is 0.25 in ROS 2 maybe someone can enlighten us about this change.