[cache] fix a cache issue with an entity created without relation cache prefill
Prefilling the relation cache has its benefits sometimes, but for
massive imports it may be a memory hog.
The caches should be kept coherent even if there was no prefill.
Closes #3828155.
HTTP cache management
=====================
.. automodule:: cubicweb.web.httpcache
Cache policies
--------------
.. autoclass:: cubicweb.web.httpcache.NoHTTPCacheManager
.. autoclass:: cubicweb.web.httpcache.MaxAgeHTTPCacheManager
.. autoclass:: cubicweb.web.httpcache.EtagHTTPCacheManager
.. autoclass:: cubicweb.web.httpcache.EntityHTTPCacheManager
Exception
---------
.. autoexception:: cubicweb.web.httpcache.NoEtag
Helper functions
----------------
.. autofunction:: cubicweb.web.httpcache.set_http_cache_headers
.. NOT YET AVAILABLE IN STABLE autofunction:: cubicweb.web.httpcache.lastmodified