Avoid NPE if run without args

[amosjyng]

Like probably many others, I didn't bother reading the documentation before running lein zprint :)

The NPE made me think the tool was broken at first. I think printing the help message instead will make it more obvious that it has to be run with actual arguments

[kkinnear]

Thanks for doing this! Good point about the NPE with no arguments. For what it is worth, I've found that the relatively recent changes to the pre-built binaries to add the -w switch and its various friends have shifted my usage of zprint from using lean zprint to using the binaries directly. Largely because they are so much faster to run. Of course, if you can't use them, then lein-zprint is fine too.

[amosjyng]

Ah interesting, I hadn't looked into zprint binaries. Though what I like about using a lein plugin is that lein downloads everything for me :)