[SPARK-16086] [SQL] fix Python UDF without arguments (for 1.6)

Fix the bug for Python UDF that does not have any arguments.

Added regression tests.

Author: Davies Liu <davies.liu@gmail.com>

Closes #13793 from davies/fix_no_arguments.

(cherry picked from commit abe36c53)
Signed-off-by: Davies Liu <davies.liu@gmail.com>