Urban Nature Access: handle undefined nodata in raster inputs

[davemfish]

Presently, if the LULC raster does not have a nodata value set, we see an error like this:

Traceback (most recent call last):
  File "natcap\invest\utils.py", line 164, in prepare_workspace
  File "invest\cli.py", line 470, in main
  File "natcap\invest\urban_nature_access.py", line 1026, in execute
  File "taskgraph\Task.py", line 674, in add_task
  File "taskgraph\Task.py", line 1093, in _call
  File "natcap\invest\urban_nature_access.py", line 1862, in _reclassify_urban_nature_area
  File "natcap\invest\utils.py", line 731, in reclassify_raster
  File "pygeoprocessing\geoprocessing.py", line 2373, in reclassify_raster
TypeError: '>' not supported between instances of 'int' and 'NoneType'

See line 1860:
urban_nature_area_map[lulc_raster_info['nodata'][0]] = FLOAT32_NODATA