backend: don't traceback when there are no files in the S3 storage
    
    When this commands returns zero:
    
        aws s3 ls s3://fedora-copr/cloudwatch/ |wc -l
    
    We get the following traceback:
    
        Traceback (most recent call last):
        File "/usr/bin/copr-aws-s3-hitcounter", line 248, in <module>
            main()
        File "/usr/bin/copr-aws-s3-hitcounter", line 216, in main
            files = s3.list_files()
        File "/usr/bin/copr-aws-s3-hitcounter", line 71, in list_files
            for obj in page["Contents"]:
        KeyError: 'Contents'
    
    Merges: #2354