tests: stop dividing by zero

[pjonsson]

Here's an excerpt of the test logs:

2025-07-25 20:36:40 [info     ] Task commencing                task=AlchemistTask(dataset=Dataset <id=7e96b76a-6b02-4427-9a1d-3c9104f2db96 product=ga_ls7e_ard_3 location=s3://dea-public-data/baseline/ga_ls7e_ard_3/102/071/2020/09/09/ga_ls7e_ard_3-1-0_102071_2020-09-09_final.stac-item.json>, settings=AlchemistSettings(specification=Specification(measurements=['nbart_blue', 'nbart_green', 'nbart_red', 'nbart_nir', 'nbart_swir_1', 'nbart_swir_2', 'oa_fmask'], transform='wofs.virtualproduct.WOfSClassifier', transform_url='https://github.com/GeoscienceAustralia/wofs/', product=None, products=['ga_ls5t_ard_3', 'ga_ls7e_ard_3', 'ga_ls8c_ard_3'], measurement_renames={'oa_fmask': 'fmask'}, transform_args={'dsm_path': 'https://dea-public-data.s3-ap-southeast-2.amazonaws.com/projects/elevation/ga_srtm_dem1sv1_0/dem1sv1_0.tif'}, transform_args_per_product=None, resampling=None, override_product_family='ard', basis='nbart_green', aws_unsigned=True), output=OutputSettings(location='/tmp/alchemist', write_data_settings={'overview_resampling': <Resampling.mode: 6>}, nodata=1, preview_image=None, preview_image_singleband={'measurement': 'water', 'lookup_table': {0: [150, 150, 110], 1: [0, 0, 0], 16: [119, 104, 87], 32: [89, 88, 86], 64: [216, 215, 214], 80: [242, 220, 180], 128: [79, 129, 189], 160: [51, 82, 119], 192: [186, 211, 242]}}, metadata={'product_family': 'wo', 'producer': 'ga.gov.au', 'dataset_version': '1.6.0', 'collection_number': '3', 'naming_conventions': 'dea_c3'}, properties={'dea:dataset_maturity': 'interim'}, reference_source_dataset=True, write_stac=True, inherit_geometry=True, explorer_url='https://explorer.dev.dea.ga.gov.au/'), processing=ProcessingSettings(dask_chunks={'x': -1, 'y': 4096}, dask_client={})))
/usr/local/lib/python3.10/dist-packages/wofs/__init__.py:1: UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.
  from pkg_resources import get_distribution, DistributionNotFound
2025-07-25 20:36:45 [info     ] Data loaded                    task=UUID('7e96b76a-6b02-4427-9a1d-3c9104f2db96')
/usr/local/lib/python3.10/dist-packages/wofs/classifier.py:81: RuntimeWarning: divide by zero encountered in divide
  c = (a - b) / (a + b)

I'm not sure if the testdata is faulty, or if the testdata is fine and wofs should stop dividing by zero, but something should be adjusted so the tests stop printing RuntimeWarning: divide by zero encountered in divide.