Mantis viewer export with img_sub_folder = None does not work

**Describe the bug**
```
plot_utils.create_mantis_dir(
fovs=subset_pixel_fovs,
mantis_project_path=os.path.join(base_dir, pixel_output_dir, "mantis"),
img_data_path=tiff_dir,
mask_output_dir=os.path.join(base_dir, pixel_output_dir, "pixel_masks"),
mapping = os.path.join(base_dir, pixel_meta_cluster_remap_name),
seg_dir=pixie_seg_dir,
mask_suffix="_pixel_mask",
seg_suffix_name=seg_suffix,
img_sub_folder=None
)
```
The function `plot_utils.create_mantis_dir` fails when `img_sub_folder = None` because it can't `os.path.join` with it then. I think in the function this needs to be added, as img_sub_folder=None is explicitly allowed in the rest of the notebook.
Thanks!