Prediction functions crash when filtered input is empty

[evancofer]

Selene encounters an error when the user provides an empty bed file as input to get_predictions_for_bed_file (or an empty fasta file for get_predictions_for_fasta_file). Specifically, the value i (as used on line 428 and 517 in model_predict.py respectively) isn't defined. Since it's unlikely that a user would provide Selene with an empty file, it is worth noting that this error is also encountered when all of the records in the input file are filtered for some reason (e.g. if none of the chromosome names in a bed file match the chromosome names in the reference genome's fasta file). Handling this case more gracefully (e.g. via explicitly raising an exception or not crashing) would be ideal.