SCIFIO_Reader: add null img check

Although the IO class no longer throws exceptions, its returned ImgPlus
can be null.

Added a check for null.
SCIFIO_Reader: check for null arg

arg is tested for being an empty string, and if so the show() command is
invoked. However, arg can be null at the point of this test. So, if it's
null or empty, we invoke show. Otherwise assume it's being run headless
as a file was passed.