Returns a single Subject's principals retrieved from the specified Realm <em>only</em> as a Collection, or an empty
Collection if there are not any principals from that realm.
<p/>
Note that this will return an empty Collection if the 'owning' subject has not yet logged in.
@param realmName the name of the Realm from which the principals were retrieved.
@return the Subject's principals from the specified Realm only as a Collection or an empty Collection if there
are not any principals from that realm.
Returns a single Subject's principals retrieved from the specified Realm <em>only</em> as a Collection, or an empty Collection if there are not any principals from that realm. <p/> Note that this will return an empty Collection if the 'owning' subject has not yet logged in.
@param realmName the name of the Realm from which the principals were retrieved. @return the Subject's principals from the specified Realm only as a Collection or an empty Collection if there are not any principals from that realm.