support null/is_null sub-ops inside has/any ops

The code in `_sub_operator` was assuming that the sub-operator dictionary argument had a `val` key, but in the case of the `null` and `is_null` operators, there is no `val` key, so we use `get` and pass `None` to `_create_operation`.

flask_restless/search.py

@@ -41,7 +41,7 @@ def _sub_operator(model, argument, fieldname):
     if isinstance(argument, dict):
         fieldname = argument['name']
         operator = argument['op']
-        argument = argument['val']
+        argument = argument.get('val')
         relation = None
         if '__' in fieldname:
             fieldname, relation = fieldname.split('__')

tests/helpers.py

@@ -200,7 +200,7 @@ class Computer(self.Base):
             owner_id = Column(Integer, ForeignKey('person.id'))
             owner = relationship('Person')
             programs = relationship('ComputerProgram',
-                                    cascade="all, delete-orphan")
+                                    cascade="all, delete-orphan", backref='computer')
 
             def speed(self):
                 return 42

tests/test_search.py

@@ -197,8 +197,12 @@ def test_has_and_any_suboperators(self):
         computer1 = self.Computer(name=u'c1', vendor=u'foo')
         computer2 = self.Computer(name=u'c2', vendor=u'bar')
         computer3 = self.Computer(name=u'c3', vendor=u'bar')
-        computer4 = self.Computer(name=u'c4', vendor=u'bar')
-        computer5 = self.Computer(name=u'c5', vendor=u'foo')
+        computer4 = self.Computer(name=u'c4', vendor=None, programs=[
+            self.ComputerProgram(program=self.Program())
+        ])
+        computer5 = self.Computer(name=u'c5', vendor=u'foo', programs=[
+            self.ComputerProgram(program=self.Program())
+        ])
         computer6 = self.Computer(name=u'c6', vendor=u'foo')
         self.session.add_all((computer1, computer2, computer3, computer4,
                               computer5, computer6))
@@ -214,11 +218,19 @@ def test_has_and_any_suboperators(self):
         d = dict(filters=[dict(name='computers', op='any', val=val)])
         result = search(self.session, self.Person, d)
         assert result.count() == 2
+        val = dict(name='vendor', op='is_null')
+        d = dict(filters=[dict(name='computers', op='any', val=val)])
+        result = search(self.session, self.Person, d)
+        assert result.count() == 1
         # test 'has'
         val = dict(name='name', op='like', val=u'%incol%')
         d = dict(filters=[dict(name='owner', op='has', val=val)])
         result = search(self.session, self.Computer, d)
         assert result.count() == 3
+        val = dict(name='vendor', op='is_null')
+        d = dict(filters=[dict(name='computer', op='has', val=val)])
+        result = search(self.session, self.ComputerProgram, d)
+        assert result.count() == 1
 
     def test_has_and_any_nested_suboperators(self):
         """Tests for the ``"has"`` and ``"any"`` operators with nested