made empty query handling use NothingNode

This is consistent with Django's SQLCompiler.

Author: timgraham

django_mongodb/compiler.py

@@ -119,8 +119,6 @@ def cursor_iter(self, cursor, chunk_size, columns):
 
     def check_query(self):
         """Check if the current query is supported by the database."""
-        if self.query.is_empty():
-            raise EmptyResultSet()
         if self.query.distinct or getattr(
             # In the case of Query.distinct().count(), the distinct attribute
             # will be set on the inner_query.

django_mongodb/query.py

@@ -8,7 +8,7 @@
 from django.db.models.lookups import Exact
 from django.db.models.sql.constants import INNER
 from django.db.models.sql.datastructures import Join
-from django.db.models.sql.where import AND, OR, XOR, WhereNode
+from django.db.models.sql.where import AND, OR, XOR, NothingNode, WhereNode
 from pymongo import ASCENDING, DESCENDING
 from pymongo.errors import DuplicateKeyError, PyMongoError
 
@@ -282,4 +282,5 @@ def where_node(self, compiler, connection):
 
 def register_nodes():
     Join.as_mql = join
+    NothingNode.as_mql = NothingNode.as_sql
     WhereNode.as_mql = where_node