Improve the format of the "dummy" cached item in BP_XProfile_XProfileData::get_data_for_user()

When a value is not found for a given field/user pair, an empty piece of data
is stored in the cache to avoid an unnecessary requery. However, the value was
not being stored with the proper format, resulting in PHP notices in certain
cases (such as when fields were preloaded into the cache for 'select'/'option'
parent/child relationships)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8524 cdf35c40-ae34-48e0-9cc9-0c9da1808c22