Don't display placeholder and image if there's no picture defined for a category

functions.php

<?php

// unload the original one
remove_action( 'woocommerce_before_subcategory_title', 'woocommerce_subcategory_thumbnail');
// load mine
add_action( 'woocommerce_before_subcategory_title', 'woocommerce_subcategory_thumbnail_custom', 10);

if ( ! function_exists( 'woocommerce_subcategory_thumbnail_custom' ) ) {
	function woocommerce_subcategory_thumbnail_custom( $category  ) {
		global $woocommerce;

		$small_thumbnail_size  = apply_filters( 'single_product_small_thumbnail_size', 'shop_catalog' );
		$image_width    = $woocommerce->get_image_size( 'shop_catalog_image_width' );
		$image_height    = $woocommerce->get_image_size( 'shop_catalog_image_height' );

		$thumbnail_id  = get_woocommerce_term_meta( $category->term_id, 'thumbnail_id', true  );

		if ( $thumbnail_id ) {
			$image = wp_get_attachment_image_src( $thumbnail_id, $small_thumbnail_size  );
			$image = $image[0];
			echo '<img src="' . $image . '" alt="' . $category->name . '" width="' . $image_width . '" height="' . $image_height . '" />';
		}

		
	}
}
?>