Fix error when calculating etag value on non-tabbedview.

[jone]

The ETagValue adapter must always return a string.

@Rotonen You have noticed None beeing returned as etag value, but I believe that this is not possible because of the only return statement:

return IDictStorage(self.published).get(storage_key) or ''

Whenever it would be None, it would return an empty string.

But I have found an error when not accessing a tabbed view:

Error in test test_default_value_is_empty_string (ftw.tabbedview.tests.test_caching.TestETagValue)
Traceback (most recent call last):
....
  File "/Users/jone/projects/packages/ftw.tabbedview/ftw/tabbedview/tests/test_caching.py", line 28, in get_etag_value_for
    return adapter()
  File "/Users/jone/projects/packages/ftw.tabbedview/ftw/tabbedview/caching.py", line 26, in __call__
    storage_key = storage_key_generator.get_key()
AttributeError: 'NoneType' object has no attribute 'get_key'

I belive that this error may have been catched and None was returned.
This change fixes this issue.

(//cc @deiferni @lukasgraf as discussed)

[Rotonen]

@jone This reliably triggers for me on the first login into a freshly installed GEVER.

Traceback (most recent call last):
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.transformchain-1.0.3-py2.7.egg/plone/transformchain/transformer.py", line 48, in __call__
    newResult = handler.transformIterable(result, encoding)
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.caching-1.0-py2.7.egg/plone/caching/hooks.py", line 144, in transformIterable
    self.mutate()
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.caching-1.0-py2.7.egg/plone/caching/hooks.py", line 169, in mutate
    operation.modifyResponse(rule, request.response)
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.app.caching-1.1.6-py2.7.egg/plone/app/caching/operations/default.py", line 143, in modifyResponse
    etag = getETagAnnotation(self.published, self.request, etags)
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.app.caching-1.1.6-py2.7.egg/plone/app/caching/operations/utils.py", line 515, in getETagAnnotation
    etag = getETag(published, request, keys=keys, extraTokens=extraTokens)
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.app.caching-1.1.6-py2.7.egg/plone/app/caching/operations/utils.py", line 546, in getETag
    value = component()
  File "/Users/joni/koodaus/git/opengever.core/eggs/ftw.tabbedview-3.7.0-py2.7.egg/ftw/tabbedview/caching.py", line 23, in __call__
    storage_key = storage_key_generator.get_key()
  File "/Users/joni/koodaus/git/opengever.core/eggs/ftw.tabbedview-3.7.0-py2.7.egg/ftw/tabbedview/defaulttab.py", line 30, in get_key
    return '-'.join(parts)
TypeError: sequence item 4: expected string, NoneType found

[Rotonen]

Something also flares up on WebDAV locks:

2017-03-10 18:58:46 ERROR root Swallowed exception in plone.caching IPubAfterTraversal event handler
Traceback (most recent call last):
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.caching-1.0-py2.7.egg/plone/caching/hooks.py", line 82, in intercept
    responseBody = operation.interceptResponse(rule, request.response)
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.app.caching-1.1.6-py2.7.egg/plone/app/caching/operations/default.py", line 102, in interceptResponse
    etag = getETagAnnotation(self.published, self.request, keys=etags)
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.app.caching-1.1.6-py2.7.egg/plone/app/caching/operations/utils.py", line 515, in getETagAnnotation
    etag = getETag(published, request, keys=keys, extraTokens=extraTokens)
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.app.caching-1.1.6-py2.7.egg/plone/app/caching/operations/utils.py", line 546, in getETag
    value = component()
  File "/Users/joni/koodaus/git/opengever.core/eggs/ftw.tabbedview-3.7.0-py2.7.egg/ftw/tabbedview/caching.py", line 22, in __call__
    IDefaultTabStorageKeyGenerator)
  File "/Users/joni/koodaus/git/opengever.core/eggs/zope.component-3.9.5-py2.7.egg/zope/component/_api.py", line 109, in getMultiAdapter
    raise ComponentLookupError(objects, interface, name)
ComponentLookupError: ((<Document at /fd/ordnungssystem/ressourcen-und-support/finanzen/planung/finanzplanung/dossier-5/document-5>, <Document at /fd/ordnungssystem/ressourcen-und-support/finanzen/planung/finanzplanung/dossier-5/document-5>, <HTTPRequest, URL=http://rotobock.local:8080/fd/ordnungssystem/ressourcen-und-support/finanzen/planung/finanzplanung/dossier-5/document-5/LOCK>), <InterfaceClass ftw.tabbedview.interfaces.IDefaultTabStorageKeyGenerator>, u'')

2017-03-10 18:59:29 ERROR root Swallowed exception in plone.caching IPubAfterTraversal event handler
Traceback (most recent call last):
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.caching-1.0-py2.7.egg/plone/caching/hooks.py", line 82, in intercept
    responseBody = operation.interceptResponse(rule, request.response)
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.app.caching-1.1.6-py2.7.egg/plone/app/caching/operations/default.py", line 102, in interceptResponse
    etag = getETagAnnotation(self.published, self.request, keys=etags)
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.app.caching-1.1.6-py2.7.egg/plone/app/caching/operations/utils.py", line 515, in getETagAnnotation
    etag = getETag(published, request, keys=keys, extraTokens=extraTokens)
  File "/Users/joni/koodaus/git/opengever.core/eggs/plone.app.caching-1.1.6-py2.7.egg/plone/app/caching/operations/utils.py", line 546, in getETag
    value = component()
  File "/Users/joni/koodaus/git/opengever.core/eggs/ftw.tabbedview-3.7.0-py2.7.egg/ftw/tabbedview/caching.py", line 22, in __call__
    IDefaultTabStorageKeyGenerator)
  File "/Users/joni/koodaus/git/opengever.core/eggs/zope.component-3.9.5-py2.7.egg/zope/component/_api.py", line 109, in getMultiAdapter
    raise ComponentLookupError(objects, interface, name)
ComponentLookupError: ((<Document at /fd/ordnungssystem/ressourcen-und-support/finanzen/planung/finanzplanung/dossier-5/document-5>, <Document at /fd/ordnungssystem/ressourcen-und-support/finanzen/planung/finanzplanung/dossier-5/document-5>, <HTTPRequest, URL=http://rotobock.local:8080/fd/ordnungssystem/ressourcen-und-support/finanzen/planung/finanzplanung/dossier-5/document-5/UNLOCK>), <InterfaceClass ftw.tabbedview.interfaces.IDefaultTabStorageKeyGenerator>, u'')

[deiferni]

👍

this probably won't fix @Rotonen problem though as in his case user.getId() seems to return None.