[SPARK-4856] [SQL] NullType instead of StringType when sampling against empty string or nul...

```
TestSQLContext.sparkContext.parallelize(
  """{"ip":"27.31.100.29","headers":{"Host":"1.abc.com","Charset":"UTF-8"}}""" ::
  """{"ip":"27.31.100.29","headers":{}}""" ::
  """{"ip":"27.31.100.29","headers":""}""" :: Nil)
```
As empty string (the "headers") will be considered as String in the beginning (in line 2 and 3), it ignores the real nested data type (struct type "headers" in line 1), and also take the line 1 (the "headers") as String Type, which is not our expected.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #3708 from chenghao-intel/json and squashes the following commits:

e7a72e9 [Cheng Hao] add more concise unit test
853de51 [Cheng Hao] NullType instead of StringType when sampling against empty string or null value