Enable running rake target without argument all

[Letme]

Now when you run rake pdf it does not build all the books, but throws some strange error (actually it builds jekyll and then fails with wrong filename). I realize that you expect to run rake pdf[all], but I think it is implied that rake pdf should be called when you want to build for all books.

It is a simple check if the book argument is null, then also consider it in all target.