fix: faulty logic update causes script to always display usage information

[bradegler]

Updates for linting changed the logic from [ "$checksum_file" = ""] which is true when there is no filename as the first argument to [[ -n "${checksum_file}" ]] which is true when there is a filename as the first argument. This is the reverse of the original logic and causes the script to always fail.

I changed the logic to be a test on the number of arguments instead of testing the length of a string in arg position 1. This will catch additional errors where callers may attempt to call this with extra arguments thinking that it can support multiple checksum files.