fix mean of widths for avg_width

[ta4tsering]

• fix the get_char_base_bboxes_and_avg_width function. Return avg_width as 0 if widths list is empty.

Input to recreate the bug: I1PD958530003.json.gz

[eroux]

looking at https://github.com/OpenPecha/Toolkit/blob/master/openpecha/formatters/ocr/ocr.py#L411 it should be None instead of -1

[eroux]

also please add a test case

[eroux]

replaced by #214