r.reclass.area: Add flag to optionally output NULL raster if no areas are left after reclass

[griembauer]

Currently, r.reclass.area with method=reclass fails if no areas greater or less than the specified value are found. This PR adds a flag to create a NULL-raster as output instead for this case.

[neteler]

(re-run black)

[neteler]

@griembauer please see the black formatting error... thanks

[wenzeslaus]

The code here needs some changes, but otherwise it is a very good improvement. I'm keeping it open, just moving the milestone.