[SPARK-4536][SQL] Add sqrt and abs to Spark SQL DSL

Spark SQL has embeded sqrt and abs but DSL doesn't support those functions.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #3401 from sarutak/dsl-missing-operator and squashes the following commits:

07700cf [Kousuke Saruta] Modified Literal(null, NullType) to Literal(null) in DslQuerySuite
8f366f8 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into dsl-missing-operator
1b88e2e [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into dsl-missing-operator
0396f89 [Kousuke Saruta] Added sqrt and abs to Spark SQL DSL

(cherry picked from commit e75e04f980281389b881df76f59ba1adc6338629)
Signed-off-by: Michael Armbrust <michael@databricks.com>