KYLIN-3699 SparkCubing failed when build with empty data

[Wayne1c]

No description provided.

[asfgit]

Can one of the admins verify this patch?

[coveralls]

Pull Request Test Coverage Report for Build 3959

• 24 of 34 (70.59%) changed or added relevant lines in 3 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.04%) to 25.847%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core-job/src/main/java/org/apache/kylin/job/JoinedFlatTable.java	0	2	0.0%
source-hive/src/main/java/org/apache/kylin/source/hive/HiveInputBase.java	0	3	0.0%
source-hive/src/main/java/org/apache/kylin/source/hive/CreateFlatHiveTableStep.java	24	29	82.76%

Files with Coverage Reduction	New Missed Lines	%
source-hive/src/main/java/org/apache/kylin/source/hive/HiveInputBase.java	1	16.82%
source-hive/src/main/java/org/apache/kylin/source/hive/CreateFlatHiveTableStep.java	1	45.95%
core-cube/src/main/java/org/apache/kylin/cube/cuboid/TreeCuboidScheduler.java	2	68.46%

Totals
Change from base Build 3957:	0.04%
Covered Lines:	17857
Relevant Lines:	69087

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💛 - Coveralls