when esi is enabled and internal uris are generated for esi-tags, an
attribute-array consisting entirely of null-values isn't handled correctly.
The reason is that php's `http_build_query()`-method outputs an empty string
for such arrays:
http_build_query(array('foo' => '')) == 'foo='
http_build_query(array('foo' => null)) == ''
In the latter case, the generation of an URI in `HttpKernel::generateInternalUri()`
generates an URI that could not be matched by the corresponding route (ex.
`_internal/Controller/.html` opposed to `_internal/Controller/none.html` which
should be expected).
This commit adds a possible solution as well as a simple test for this issue.
The only missing part is ContainerAwareEventManager::addEventSubscriberService(),
because I'm not sure how to find out the class name of a service in the DIC.
Also, inline documentation of this code needs to be finished once it is accepted.
