[SPARK-2908] [SQL] JsonRDD.nullTypeToStringType does not convert all NullType to StringType

JIRA: https://issues.apache.org/jira/browse/SPARK-2908

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1840 from yhuai/SPARK-2908 and squashes the following commits:

86e833e [Yin Huai] Update test.
cb11759 [Yin Huai] nullTypeToStringType should check columns with the type of array of structs.

(cherry picked from commit 0489cee6b24ca34f1adab03a75d157e04a9e06b7)
Signed-off-by: Michael Armbrust <michael@databricks.com>