Fix AttributeError when handle periodic job with github driver

Now, if we config periodic pipeline with github driver, the periodic jobs
won't be triggered and an AttributeError will be raised. That is because
the TimerTriggerEvent object don't have "commits" attribute which is used
in the zuul.driver.github.githubconnection.getPushedFileNames method.
We don't need the attribute assignment in periodic job, so this change
skipp it.

Change-Id: Ie1e85cc57ada17eacc44fb084f5f3f17aa8e9c45
diff --git a/tests/fixtures/layouts/timer-github.yaml b/tests/fixtures/layouts/timer-github.yaml
new file mode 100644
index 0000000..4f3efe4
--- /dev/null
+++ b/tests/fixtures/layouts/timer-github.yaml
@@ -0,0 +1,25 @@
+- pipeline:
+    name: periodic
+    manager: independent
+    trigger:
+      timer:
+        - time: '* * * * * */1'
+
+- job:
+    name: base
+    parent: null
+    run: playbooks/base.yaml
+
+- job:
+    name: project-bitrot
+    nodeset:
+      nodes:
+        - name: static
+          label: ubuntu-xenial
+    run: playbooks/project-bitrot.yaml
+
+- project:
+    name: org/project
+    periodic:
+      jobs:
+        - project-bitrot