fcm-manage-trac-env-session: fix session logic

If a user already has some entries the `session_attribute` table, the
old logic will not insert a new `name` or `email` for the user. The new
logic will only skip the `name` or `email` attribute if already set
correctly.
Merge pull request #162 from matthewrmshin/fix-trac-session-logic

fcm-manage-trac-env-session: fix session logic