Fix infinite loop if fasta is empty

[rzlim08]

• If the incoming bam file has no alignments to the database, the fasta file is empty
• Since the fasta file is empty, list_size is 0
• passing a list_size of 0 to chunk_list gets stuck in an infinite loop
• This PR returns an empty list of lists; the rest of the code seems well defined enough to output appropriate empty outputs

[raphenya]

@rzlim08 Thank you. good catch.