only show <devices> if there are devices (bug 639899)

apps/blocklist/templates/blocklist/blocklist.xml

@@ -50,11 +50,13 @@
     <gfxBlacklistEntry>
       <os>{{ gfx.os }}</os>
       <vendor>{{ gfx.vendor }}</vendor>
-      <devices>
-        {% for device in gfx.devices.split(' ') %}
-          <device>{{ device }}</device>
-        {% endfor %}
-      </devices>
+      {% if gfx.devices %}
+        <devices>
+          {% for device in gfx.devices.split(' ') %}
+            <device>{{ device }}</device>
+          {% endfor %}
+        </devices>
+      {% endif %}
       <feature>{{ gfx.feature }}</feature>
       <featureStatus>{{ gfx.feature_status }}</featureStatus>
       <driverVersion>{{ gfx.driver_version }}</driverVersion>

apps/blocklist/tests.py

@@ -306,3 +306,9 @@ class BlocklistGfxTest(BlocklistTest):
         for device, val in zip(devices.getElementsByTagName('device'),
                                self.gfx.devices.split(' ')):
             eq_(device.childNodes[0].wholeText, val)
+
+    def test_empty_devices(self):
+        self.gfx.devices = None
+        self.gfx.save()
+        r = self.client.get(self.fx4_url)
+        self.assertNotContains(r, '<devices>')