[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.

bin/compute-classpath.sh

@@ -25,7 +25,11 @@ FWDIR="$(cd "`dirname "$0"`"/..; pwd)"
 
 . "$FWDIR"/bin/load-spark-env.sh
 
-CLASSPATH="$SPARK_CLASSPATH:$SPARK_SUBMIT_CLASSPATH"
+if [ -n "$SPARK_CLASSPATH" ]; then
+  CLASSPATH="$SPARK_CLASSPATH:$SPARK_SUBMIT_CLASSPATH"
+else
+  CLASSPATH="$SPARK_SUBMIT_CLASSPATH"
+fi
 
 # Build up classpath
 if [ -n "$SPARK_CONF_DIR" ]; then