[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>

itests/org.openhab.binding.modbus.tests/itest.bndrun

@@ -27,8 +27,6 @@ Fragment-Host: org.openhab.binding.modbus
 #
 -runbundles: \
 	javax.measure.unit-api;version='[1.0.0,1.0.1)',\
-	org.apache.commons.io;version='[2.2.0,2.2.1)',\
-	org.apache.commons.lang;version='[2.6.0,2.6.1)',\
 	org.apache.felix.configadmin;version='[1.9.8,1.9.9)',\
 	org.apache.felix.scr;version='[2.1.10,2.1.11)',\
 	org.eclipse.equinox.event;version='[1.4.300,1.4.301)',\