report message even if no action is specified

Whenever a pipeline does not provide any action, the job message would
not be reported which is a bit annoying.  This patch makes it so we skip
reporting when there is no action and no message.

My use case is:

A `postmerge` pipeline that simply react to the `change-merged` events,
since the change is closed, there is no point in voting on it.  Jobs
then uses the `success-message` and `failure-message` to finely tune
what is reported back to the user.

Configuration exemple

  pipelines:
   - name: postmerge
     manager: IndependentPipelineManager
     trigger:
      - event: change-merged

  projects:
   - name: operations/config
     postmerge:
      - operations-config-deploy

  jobs:
   - name: operations-config-deploy
     branch: ^production$
     success-message: 'Your configuration has been deployed by Jenkins.'
     failure-message: 'Configuration NOT APPLIED. You broke wikipedia.'

Before this patch, the debug would be:

------------8<---------------8<---------------8<-----------------------
INFO zuul.IndependentPipelineManager: Reporting
change <Change 0x7f67a0d61f50 61980,1>, action: None

DEBUG zuul.Gerrit: Report change <Change
0x7f67a0d61f50 61980,1>, action None, message: Build succeeded.

- https://jenkins-url-goes-there.example.com/
  : Your configuration has been deployed by Jenkins.

DEBUG zuul.Gerrit: No action specified; not reporting

DEBUG zuul.IndependentPipelineManager: Removing reported
------------8<---------------8<---------------8<-----------------------

Change-Id: I9ab4d47a8cfc1ae66870e8ac55e8f78dfc29da3d
Reviewed-on: https://review.openstack.org/28045
Reviewed-by: Jesse Keating <jesse.keating@rackspace.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins