[SPARK-8782] [SQL] Fix code generation for ORDER BY NULL

This fixes code generation for queries containing `ORDER BY NULL`.  Previously, the generated code would fail to compile.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7179 from JoshRosen/generate-order-fixes and squashes the following commits:

6ef49a6 [Josh Rosen] Fix ORDER BY NULL
0036696 [Josh Rosen] Add regression test for SPARK-8782 (ORDER BY NULL)