Don't prematurely start Symfony session

I removed the calls to `session->has()` and `session->set()` in the constructor of `SymfonySession`, since it may prematurely start the session.

The calls are not necessary anyway, since it's possible to return a default value if the variable is not found in the session (via the second argument of `get()`). Although I opted to use `is_array()` checks instead since it's possible that a malformed value may be stored in the session.