if DYLD_FALLBACK_LIBRARY_PATH is empty, use default from `man dyld`

I believe this fixes https://github.com/boxen/puppet-java/issues/23

When `$DYLD_FALLBACK_LIBRARY_PATH` is empty, the default path is used internally. From `man dyld`

>       DYLD_FALLBACK_LIBRARY_PATH
              This is a colon  separated  list  of  directories  that  contain
              libraries.  It is used as the default location for libraries not
              found  in  their  install  path.   By  default,  it  is  set  to
              $(HOME)/lib:/usr/local/lib:/lib:/usr/lib.

By always appending homebrew's lib path to `DYLD_FALLBACK_LIBRARY_PATH` we end up breaking compatibility between java's ability to correctly reference the default paths for C libs that are required to execute some java, including `jruby`.

Further Reading:
http://stackoverflow.com/questions/3146274/is-it-ok-to-use-dyld-library-path-on-mac-os-x-and-whats-the-dynamic-library-s#comment21758755_3172515
https://github.com/Homebrew/homebrew/issues/13463