[bfss, migration] fix storage migration (closes #2189206) The storage migration intend to clean the attributes cache but in practice it was trying to discard attribut edition. As no attribut were edited this lead to a crash.

msgid ""
msgstr ""
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"Generated-By: pygettext.py 1.5\n"
"Plural-Forms: nplurals=2; plural=(n > 1);\n"