Filter empty args before passing to os.execvp.

This PR is an attempt to fix https://github.com/nathanmarz/storm/issues/657.

A `java.lang.NoClassDefFoundError` exception is thrown if `JAR_JVM_OPTS` is empty. This PR filters empty arguments before passing `all_args` to `os.execvp`.  The functionality intended by https://github.com/nathanmarz/storm/commit/0fca3034ff90c8d5de2e327d58085580fcc74256 should still remain.