Fix to dev.plone.org ticket #13514: Law of Demeter when profile_id is None

More details here: https://dev.plone.org/ticket/13514

Essentially, when doing a simple import of an export using *Products.CMFCore.exportimport.content.exportSiteStructure*, error is encountered because **profile_id** is set to **None**, so Law of Demeter is broken and *AttributeError* thrown.

Solution commit is similar to that of the Import Event Handler within [*Products.GenericSetup.events.handleProfileImportedEvent*](http://zope3.pov.lt/trac/browser/Products.GenericSetup/trunk/Products/GenericSetup/events.py#L47)

```
2013-04-08 15:39:09 ERROR Zope.SiteErrorLog 1365399549.760.408159064119 http://localhost:8080/Site/portal_setup/manage_importTarball
Traceback (innermost last):
  Module ZPublisher.Publish, line 126, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 46, in call_object
  Module Products.GenericSetup.tool, line 565, in manage_importTarball
  Module Products.GenericSetup.tool, line 350, in runAllImportStepsFromProfile
  Module Products.GenericSetup.tool, line 1080, in _runImportStepsFromContext
  Module zope.event, line 31, in notify
  Module zope.component.event, line 24, in dispatch
  Module zope.component._api, line 136, in subscribers
  Module zope.component.registry, line 321, in subscribers
  Module zope.interface.adapter, line 585, in subscribers
  Module Products.CMFPlone.events, line 24, in profileImportedEventHandler
AttributeError: 'NoneType' object has no attribute 'replace'
```