get_boxes: exception needed when no object is detected at the particular object threshold given

[dlusseau]

Describe the bug
at the moment if there is no object detected in an image at a particular obj_threshold, then get_boxes throws the following error:

"Error: nm must be NULL or a character vector the same length as 'x'"

this is because the fixed name array given to set_names in clean_boxes is not the same length as the values array (there are no coordinates available for that particular image).
#line 172 of yolo3_predict.R

I suggest subsetting out the 'empty' images between lines 32 and 33 to treat them separately (return "none" label for example?)