ImageConverter: only call setSeries on non-negative values (rebased onto develop)



This is the same as gh-740 but rebased onto develop.

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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```