[bfss] Make ``current_fs_path`` return None when an attribute has no value current_fs_path was returning the future fspath when no value was defined. * This does not make any sense * This tricks the storage into believing that updating the attribute content is not necessary. The file was written but the attributes was not updated. as a result the assignement was not made. (close #1725446)

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile