Examples: Access the default VHost not possible

**Describe the bug**
Its not possible to access the default vhost with e.g. `rmqhelloworld_producer`

**To Reproduce**
Steps to reproduce the behavior:
1. Call `rmqhelloworld_producer amqp://guest:guest@localhost:5672/`
2. See Error: `Error: Connection error NOT_ALLOWED - vhost not found, rc: 530`

**Expected behavior**
It would not use "" as vhost but rather the default "/" vhost

**Additional context**
Could be fixed in `ConnectionString::parseParts` function or Within the getter `SimpleEndpoint::vhost` by using `/` as vhost if it is empty.