[1.6.x] Fixed #21249 -- variable name typo in compiler.get_grouping()

The typo could have consequences in exceptional cases, but I didn't
figure out a way to actually produce such a case, so not tests.

Report & patch by Michael Manfre.

Backport of 86c248aa64 from master

django/db/models/sql/compiler.py

@@ -598,7 +598,7 @@ class SQLCompiler(object):
                 for order, order_params in ordering_group_by:
                     # Even if we have seen the same SQL string, it might have
                     # different params, so, we add same SQL in "has params" case.
-                    if order not in seen or params:
+                    if order not in seen or order_params:
                         result.append(order)
                         params.extend(order_params)
                         seen.add(order)