remove unnecessary ternary in where_node()

Author: WaVEV

django_mongodb/query.py

@@ -289,7 +289,7 @@ def where_node(self, compiler, connection):
  if len(children_mql) == 1:
  mql = children_mql[0]
  elif len(children_mql) > 1:
- mql = {operator: children_mql} if children_mql else {}
+ mql = {operator: children_mql}
  else:
  mql = {}