fix(downsample): check num_mips is not none before comparison

igneous/task_creation/image.py

@@ -249,7 +249,7 @@ def ds_shape(mip, chunk_size=None, factor=None):
  memory_target, vol.dtype, shape, factor
  )
 
- if viable_mips < num_mips:
+ if num_mips is not None and viable_mips < num_mips:
  raise ValueError(
  f"Memory limit ({memory_target} bytes) too low to "
  "compute {num_mips} mips at a time. {viable_mips} mips possible.")