i.sentinel.mask: fix no clouds/shadow overlap error

[griembauer]

Running i.sentinel.mask, I encountered

Traceback (most recent call last):
  File "/home/griembauer/.grass8/addons/scripts/i.sentinel.mask", line 780, in <module>
    main()
  File "/home/griembauer/.grass8/addons/scripts/i.sentinel.mask", line 708, in main
    compute_shadow_shift(clouds, dark_pixels, sun_zenith, sun_azimuth)
  File "/home/griembauer/.grass8/addons/scripts/i.sentinel.mask", line 438, in compute_shadow_shift
    get_overlap(TMP_NAME, dark_pixel_map, map_region, new_region)
  File "/home/griembauer/.grass8/addons/scripts/i.sentinel.mask", line 361, in get_overlap
    gs.read_command(
IndexError: list index out of range

which corresponds to

    # measure overlap
    overlap = int(
        gs.read_command(
            "r.stats",
            quiet=True,
            flags="cn",
            input=f"{clouds},{dark_pixels}",
            separator=",",
        )
        .strip()
        .split(",")[2]    <---------
        .strip()
    )

Tested:

(Pdb) gs.read_command("r.stats",quiet=True, flags="cn",input=f"{clouds},{dark_pixels}",separator=",",)
 100%
''

I assume this occurs when the maps clouds and dark_pixels don't have any overlap. They contain only the values NoData and 0, so due to the -nflag, r.stats does not give any information on category combinations if the only occuring category combinations contain at least one NoData.

This PR fixes this issue, but I am not sure if this is consistent with the remaining logic of the module