Don't render None as "null"

In a config file you never want the string "null" in place of a null
value. Just render an empty string instead.

Change-Id: Ibe145c4929d4944cd8d51c07a64aee6fd5f831f2
Closes-Bug: #1830967
Story: 2005797
(cherry picked from commit 75d527ab8f)

os_apply_config/renderers.py

@@ -38,4 +38,6 @@ class JsonRenderer(pystache.Renderer):
                                                   partials, missing_tags)
 
     def str_coerce(self, val):
+        if val is None:
+            return b''
         return json.dumps(val)

os_apply_config/tests/test_apply_config.py

@@ -337,6 +337,11 @@ class OSConfigApplierTestCase(testtools.TestCase):
     def test_render_moustache_bad_key(self):
         self.assertEqual(u'', apply_config.render_moustache("{{badkey}}", {}))
 
+    def test_render_moustache_none(self):
+        self.assertEqual('foo: ',
+                         apply_config.render_moustache("foo: {{foo}}",
+                                                       {'foo': None}))
+
     def test_render_executable(self):
         params = {"x": "foo"}
         self.assertEqual("foo\n", apply_config.render_executable(