It looks like the occupancy map is created from the lidar. To fill empty space, you need to enable "Grid/RayTracing: true". On less powerful computers, you can set "Grid/3D: false".
See also other Grid related parameters with:
rtabmap --params | grep Grid
In particular with ring-lidar like this, it could be just easier to threshold the floor with "Grid/NormalsSegmentation: false" and "Grid/MaxGroundHeight: 0.05".
cheers,
Mathieu