[modbus] fix defaults for tcp and serial things and some other minor cleanup (#10147)

[modbus] fix defaults for tcp and serial things and some other minor cleanup (#10147)

* [modbus] More strict nullness. Remove apache.commons.lang from itests
* [modbus] Defaults for tcp and serial things according to docs
* [modbus] further explicit defaults
* [modbus] document default encoding for serial.
RTU is pretty much the only one used in the field.
Previous default was ascii implicitly.
* [modbus] verify defaults are used for undefined configuration parameters

Signed-off-by: Sami Salonen <ssalonen@gmail.com>