Fix for #486

[alecpm]

This fixes a long standing and serious issue with the summary view from v1.4.1 where disabling thumbnails fro the Site control panel would cause it to thow a traceback:

Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 162, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 371, in publish_module
  Module ZPublisher.WSGIPublisher, line 266, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 63, in call_object
  Module zope.browserpage.simpleviewclass, line 41, in __call__
  Module Products.Five.browser.pagetemplatefile, line 126, in __call__
  Module Products.Five.browser.pagetemplatefile, line 58, in __call__
  Module zope.pagetemplate.pagetemplate, line 133, in pt_render
  Module Products.PageTemplates.engine, line 378, in __call__
  Module z3c.pt.pagetemplate, line 176, in render
  Module chameleon.zpt.template, line 302, in render
  Module chameleon.template, line 214, in render
  Module chameleon.utils, line 53, in raise_with_traceback
  Module chameleon.template, line 192, in render
  Module 21c4cf128bad7545198495d7cb899d31, line 650, in render
  Module 2d1571356263e8cd1c946fe23f300f29, line 860, in render_master
  Module 2d1571356263e8cd1c946fe23f300f29, line 1489, in render_content
  Module 21c4cf128bad7545198495d7cb899d31, line 635, in __fill_content_core
  Module 21c4cf128bad7545198495d7cb899d31, line 387, in render_content_core
  Module 720ac2df5e24b26392deaed5ddf34996, line 122, in render_content_core
  Module 720ac2df5e24b26392deaed5ddf34996, line 505, in render_listing
  Module 21c4cf128bad7545198495d7cb899d31, line 360, in __fill_entries
  Module 720ac2df5e24b26392deaed5ddf34996, line 952, in render_entries
  Module 21c4cf128bad7545198495d7cb899d31, line 196, in __fill_entry
  Module zope.tales.pythonexpr, line 73, in __call__
   - __traceback_info__: (item_url + '/@@images/image/' + thumb_scale_summary)
  Module <string>, line 1, in <module>
TypeError: can only concatenate str (not "NoneType") to str

I made this PR against the 2.2.x branch because that's the code I need it for. It should apply cleanly to 1.4.x and straighforwardly to master.

[mister-roboto]

@alecpm thanks for creating this Pull Request and help improve Plone!

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass.

Whenever you feel that the pull request is ready to be tested, either start all jenkins jobs pull requests by yourself, or simply add a comment in this pull request stating:

@jenkins-plone-org please run jobs

With this simple comment all the jobs will be started automatically.

Happy hacking!

[ale-rt]

Thanks!

[alecpm]

@jenkins-plone-org please run jobs

[ale-rt]

@jenkins-plone-org please run jobs

[ale-rt]

@jenkins-plone-org please run jobs