How do you set values to 0 on unused adc channels ?

Question

I am trying to setup the following pins A1 (Pin 0.05) , A2 (Pin 0.06) , A3(Pin 0.07) & A4 (Pin 0.25) to read adc values using the nrf5340dk board. I have a joystick module connected to pins A1 & A2 and nothing is connected to pins A3 & A4 but for some reason i am still seeing values on the nrf terminal , i dont know what these values are and how can i setup the channels so that if nothing is connected i read only 0's ?

My code is shown below:

/*
 * Copyright (c) 2020 Libre Solar Technologies GmbH
 *
 * SPDX-License-Identifier: Apache-2.0
 */

#include <inttypes.h>
#include <stddef.h>
#include <stdint.h>

#include <zephyr/device.h>
#include <zephyr/devicetree.h>
#include <zephyr/drivers/adc.h>
#include <zephyr/kernel.h>
#include <zephyr/sys/printk.h>
#include <zephyr/sys/util.h>
#include <zephyr/drivers/gpio.h>

#if !DT_NODE_EXISTS(DT_PATH(zephyr_user)) || \
	!DT_NODE_HAS_PROP(DT_PATH(zephyr_user), io_channels)
#error "No suitable devicetree overlay specified"
#endif

#define DT_SPEC_AND_COMMA(node_id, prop, idx) \
	ADC_DT_SPEC_GET_BY_IDX(node_id, idx),

/* Data of ADC io-channels specified in devicetree. */
static const struct adc_dt_spec adc_channels[] = {
	DT_FOREACH_PROP_ELEM(DT_PATH(zephyr_user), io_channels,
			     DT_SPEC_AND_COMMA)
};

void main(void)
{
	int err;
	int16_t buf;
	struct adc_sequence sequence = {
		.buffer = &buf,
		/* buffer size in bytes, not number of samples */
		.buffer_size = sizeof(buf),
	};

	/* Configure channels individually prior to sampling. */
	for (size_t i = 0U; i < ARRAY_SIZE(adc_channels); i++) {
		if (!device_is_ready(adc_channels[i].dev)) {
			printk("ADC controller device not ready\n");
			return;
		}

		err = adc_channel_setup_dt(&adc_channels[i]);
		if (err < 0) {
			printk("Could not setup channel #%d (%d)\n", i, err);
			return;
		}
	}

	while (1) {
		//printk("ADC reading:\n");
			/*---------Channel A1 (Pin 0.04)--------*/
			printk("\n channel %d: ",
			       adc_channels[0].channel_id);
			(void)adc_sequence_init_dt(&adc_channels[0], &sequence);

			err = adc_read(adc_channels[0].dev, &sequence);
			printk("%"PRId16,buf);


			/*---------Channel A2 (Pin 0.06)--------*/
			printk("\n channel %d: ",
			       adc_channels[1].channel_id);
			(void)adc_sequence_init_dt(&adc_channels[1], &sequence);

			err = adc_read(adc_channels[1].dev, &sequence);
			printk("%"PRId16,buf);

			/*---------Channel A3 (Pin 0.07)--------*/
			printk("\n channel %d: ",
			       adc_channels[2].channel_id);
			(void)adc_sequence_init_dt(&adc_channels[2], &sequence);

			err = adc_read(adc_channels[2].dev, &sequence);
			printk("%"PRId16,buf);

			/*---------Channel A4 (Pin 0.25)--------*/
			printk("\n channel %d: ",
			       adc_channels[3].channel_id);
			(void)adc_sequence_init_dt(&adc_channels[3], &sequence);

			err = adc_read(adc_channels[3].dev, &sequence);
			printk("%"PRId16,buf);


		

		k_sleep(K_MSEC(1000));
	}
}

nrf5340dk_nrf5340_cpuapp.overlay

/*
 * SPDX-License-Identifier: Apache-2.0
 *
 * Copyright (c) 2022 Nordic Semiconductor ASA
 */


/ {
	zephyr,user {
		io-channels = <&adc 1>, <&adc 2>, <&adc 3> , <&adc 4> ;
	};
};

&adc {
	#address-cells = <1>;
	#size-cells = <0>;

	[email protected] {
		reg = <1>;
		zephyr,gain = "ADC_GAIN_1_4";
		zephyr,reference = "ADC_REF_INTERNAL";
		zephyr,acquisition-time = <ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 20)>;
		zephyr,input-positive = <NRF_SAADC_AIN1>; /* P0.05 */
		zephyr,resolution = <12>;
	};

	[email protected] {
		reg = <2>;
		zephyr,gain = "ADC_GAIN_1_4";
		zephyr,reference = "ADC_REF_INTERNAL";
		zephyr,acquisition-time = <ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 20)>;
		zephyr,input-positive = <NRF_SAADC_AIN2>;/* P0.06 */
		zephyr,resolution = <12>;
	};

	[email protected] {
		reg = <3>;
		zephyr,gain = "ADC_GAIN_1_4";
		zephyr,reference = "ADC_REF_INTERNAL";
		zephyr,acquisition-time = <ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 20)>;
		zephyr,input-positive = <NRF_SAADC_AIN3>; /* P0.07 */
		zephyr,resolution = <12>;
	};


	[email protected] {
		reg = <4>;
		zephyr,gain = "ADC_GAIN_1_4";
		zephyr,reference = "ADC_REF_INTERNAL";
		zephyr,acquisition-time = <ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 20)>;
		zephyr,input-positive = <NRF_SAADC_AIN4>; /* P0.07 */
		zephyr,resolution = <12>;
	};
};

prj.conf

CONFIG_ADC=y