Fix NPE in ShrunkDict step

There are some fields in CreateShrunkenDictionary(a spark function) that can not be serialized, causing NPE.

Caused by: java.lang.NullPointerException
at org.apache.kylin.common.KylinConfig.getManager(KylinConfig.java:474)
at org.apache.kylin.common.KylinConfig.getManager(KylinConfig.java:472)
at org.apache.kylin.cube.CubeDescManager.getInstance(CubeDescManager.java:67)
at org.apache.kylin.cube.CubeInstance.getDescriptor(CubeInstance.java:212)
at org.apache.kylin.cube.CubeSegment.getCubeDesc(CubeSegment.java:142)
at org.apache.kylin.cube.CubeSegment.buildGlobalDictionaryMap(CubeSegment.java:386)
at org.apache.kylin.engine.spark.SparkCubingByLayer$CreateShrunkenDictionary.call(SparkCubingByLayer.java:592)
at org.apache.kylin.engine.spark.SparkCubingByLayer$CreateShrunkenDictionary.call(SparkCubingByLayer.java:519)