Catalog can't be empty

If the user leaves the catalog information empty, driver gets HTTP 400 on subsequent operations as Presto doesn't allow that (see https://github.com/facebook/presto/blob/e9a9eeb50e0511d90295ad9459fed802af355666/presto-main/src/main/java/com/facebook/presto/server/StatementResource.java#L157)

This change also puts the initialization of the defaults in driver.d:parseConnectionString
Ah, I see why you did that now. That was a remnant from way way back. I actually killed that default value in the socks diff I'm working on.