uri: don't create params table in parse if there's no params

uri.parse always creates params table even if there's no query
parameters in the given uri:

tarantool> require('uri').parse('localhost:3301')
---
- host: localhost
  service: '3301'
  params: []
...

This is confusing and inconsistent with other uri properties, which are
created only if they actually exist. Let's create the params table only
if the given uri actually has any params.