Filter the social icons block.

social-icons.php

<?php
/**
 * Filter the social icons block.
 *
 * @param string $block_content The block content.
 * @param array  $block         The block.
 *
 * @return string
 */
function filter_social_icons_block( $block_content, $block ) {

  // Grab our options (note that this could also come from an ACF options field).
	$settings = get_option( 'my_options' ); 

	if ( 'twitter' === $block['attrs']['service'] && '' !== $settings['twitter'] ) {
		$new_content = new WP_HTML_Tag_Processor( $block_content );
		if ( $new_content->next_tag( 'a' ) ) {
			$new_content->set_attribute( 'href', $settings['twitter'] );
		}
		$block_content = $new_content->get_updated_html();
	}
	if ( 'facebook' === $block['attrs']['service'] && '' !== $settings['facebook'] ) {
		$new_content = new WP_HTML_Tag_Processor( $block_content );
		if ( $new_content->next_tag( 'a' ) ) {
			$new_content->set_attribute( 'href', $settings['facebook'] );
		}
		$block_content = $new_content->get_updated_html();
	}
	if ( 'instagram' === $block['attrs']['service'] && '' !== $settings['instagram'] ) {
		$new_content = new WP_HTML_Tag_Processor( $block_content );
		if ( $new_content->next_tag( 'a' ) ) {
			$new_content->set_attribute( 'href', $settings['instagram'] );
		}
		$block_content = $new_content->get_updated_html();
	}

	return $block_content;
}
add_filter( 'render_block_core/social-link', 'filter_social_icons_block', 10, 2 );