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
Possible that I'm missing something again, but it looks like this is redundant to line 102/108? Just change that line so that it offers a default value, `getOrDefault("PRESTOSCHEMA", "tiny");`, but I'm not sure that this is a valuable default.