SPARK-6245 [SQL] jsonRDD() of empty RDD results in exception

Avoid `UnsupportedOperationException` from JsonRDD.inferSchema on empty RDD.

Not sure if this is supposed to be an error (but a better one), but it seems like this case can come up if the input is down-sampled so much that nothing is sampled.

Now stuff like this:
```
sqlContext.jsonRDD(sc.parallelize(List[String]()))
```
just results in
```
org.apache.spark.sql.DataFrame = []
```

Author: Sean Owen <sowen@cloudera.com>

Closes #4971 from srowen/SPARK-6245 and squashes the following commits:

3699964 [Sean Owen] Set() -> Set.empty
3c619e1 [Sean Owen] Avoid UnsupportedOperationException from JsonRDD.inferSchema on empty RDD

0 files changed, 0 insertions, 0 deletions