getPkgs: Fix return empty return value

GRANBase:::getPkgs returns an empty list for 'attached' and 'loaded' packages if none is present in the session info.  This should be the case for a vanilla R.

The 'parsedSessionInfo' class expects a 'data.frame' for the 'attached' and 'loaded' slots, which results in an error.

The patch changes the return value from a list to a data.frame.