ImageConverter: only call setSeries on non-negative values

This was introduced in 370dfdf2b4b6e072ff3a20264bbcb8a722ec8c37 (i.e. gh-728).
Without this change, bfconvert will throw an IllegalArgumentException
unless the '-series' flag is used.

To test, verify that the tests from gh-728 pass, in addition to ```bfconvert input-file output-file.tiff```