Fix null images

[zhiqiangdon]

Issue #, if available:

Description of changes:

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[tonyhoo]

This will convert the None value to ["None"], will that introduce issue during training as the image can't be loaded or we will just ignore invalid path later on

[sxjscience]

It will be guarded by

autogluon/multimodal/src/autogluon/multimodal/data/process_image.py

Lines 359 to 365 in 70dd543

	try:
	if feature_modalities.get(per_col_name) == IMAGE_BYTEARRAY:
	image_feature = BytesIO(img_feature)
	else:
	image_feature = img_feature
	with PIL.Image.open(image_feature) as img:
	img = img.convert(image_mode)

[zhiqiangdon]

We would try to open the images and skip them if failures happen.

[sxjscience]

LGTM as a simple fix. However, we will need to handle/test missing values for all column types (including ROI / entity). Also CC @FANGAreNotGnu @cheungdaven

[github-actions]

Job PR-2647-acb6764 is done.
Docs are uploaded to http://autogluon-staging.s3-website-us-west-2.amazonaws.com/PR-2647/acb6764/index.html