fix: panic when transport is not initialized

[EJTJ3]

This PR resolves a runtime panic that occurs when the Modbus transport fails to initialize. Previously, if the transport was not successfully created, attempting to read the registers would lead to a runtime panic. This fix ensures that the code checks for a valid transport before proceeding with register reads, preventing the panic and providing a error message instead.

How to reproduce

package main

import (
	"time"

	"github.com/simonvetter/modbus"
)

func main() {
	modbusClient, err := modbus.NewClient(&modbus.ClientConfiguration{
		URL:     "tcp://192.168.2.2:502", // Some fake URL 
		Timeout: time.Second,
	})

	if err = modbusClient.Open(); err != nil {
		println(err.Error())
	}

	if _, err = modbusClient.ReadRegister(100, modbus.HOLDING_REGISTER); err != nil {
		println(err.Error())
	}
}

Output without the fix:

dial tcp 192.168.2.2:502: i/o timeout
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x20 pc=0x1022d2064]

Output with the fix:

dial tcp 192.168.2.2:502: i/o timeout
Transport is not initialized