[SPARK-10772] [STREAMING] [SCALA] NullPointerException when transform function in DStream returns NULL

Currently, the ```TransformedDStream``` will using ```Some(transformFunc(parentRDDs, validTime))``` as compute return value, when the ```transformFunc``` somehow returns null as return value, the followed operator will have NullPointerExeception.

This fix uses the ```Option()``` instead of ```Some()``` to deal with the possible null value. When   ```transformFunc``` returns ```null```, the option will transform null to ```None```, the downstream can handle ```None``` correctly.

NOTE (2015-09-25): The latest fix will check the return value of transform function, if it is ```NULL```, a spark exception will be thrown out

Author: Jacker Hu <gt.hu.chang@gmail.com>
Author: jhu-chang <gt.hu.chang@gmail.com>

Closes #8881 from jhu-chang/Fix_Transform.

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