Fix several emqx_bridge issues:

- fix Cassandra bridge connect error occurring when the bridge is configured without username/password 
  (Cassandra doesn't require user credentials when it is configured with `authenticator: AllowAllAuthenticator`)
- fix SQL Server bridge connect error caused by an empty password
- make `username` a required field in Oracle bridge
- fix IoTDB bridge error caused by setting base URL without scheme (e.g. `<host>:<port>`)