[SPARK-4831] Do not include SPARK_CLASSPATH if empty

My guess for fixing https://issues.apache.org/jira/browse/SPARK-4831.

Author: Daniel Darabos <darabos.daniel@gmail.com>

Closes #3678 from darabos/patch-1 and squashes the following commits:

36e1243 [Daniel Darabos] Do not include SPARK_CLASSPATH if empty.