You can check if the quantity is less than 1, then the minimum quantity must be considered 1. Please check the following code.

/* Display the total weight and price in the mini cart shopping cart widget footer */
function display_mini_cart_total_weight() {
    if ( ! WC()->cart->is_empty() ) {
        $total_weight = 0;
        $total_price = 0;

        foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
            $weight = 0;
            $price = 0;
            $product = $cart_item['data'];
            $variation_id = $cart_item['variation_id'];

            if ( $variation_id ) {
                // Get the selected variation
                $variation = wc_get_product( $variation_id );

                if ( $variation ) {
                    // Get the weight and price of the variation
                    $weight = $variation->get_weight();
                    $price = $variation->get_price();
                }
            } else {
                // Get the weight and price of the product
                $weight = $product->get_weight();
                $price = $product->get_price();
            }

            $quantity = $cart_item['quantity'];

            if( $quantity < 1 ){
                $quantity = 1;
            }

            // Calculate the weight and price of the current product
            $product_weight = $weight * $quantity;
            $product_price = $price * $quantity;

            // Add the product's weight and price to the total weight and price
            $total_weight += $product_weight;
            $total_price += $product_price;
        }

        // Output the total weight and price in the mini cart shopping cart widget footer
        $total_weight_display = $total_weight . ' Kg'; // Append ' Kg' to the total weight
        $total_price_display = wc_price( $total_price ); // Format the total price as WooCommerce price

        echo '<tr class="total-weight-row">
                <td colspan="3" class="total-weight-cell">
                    <p class="total-weight-label woocommerce-mini-cart__total">' . __('Total Weight:', 'chahar-4-rahewordpress') . '</p>
                    <p class="total-weight-value woocommerce-Price-amount amount">' . $total_weight_display . '</p>
                </td>
            </tr>';

        echo '<tr class="total-price-row">
                <td colspan="3" class="total-price-cell">
                    <p class="total-price-label woocommerce-mini-cart__total">' . __('Total Price 1:', 'chahar-4-rahewordpress') . '</p>
                    <p class="total-price-value woocommerce-Price-amount amount">' . $total_price_display . '</p>
                </td>
            </tr>';
    }
}

add_action( 'woocommerce_widget_shopping_cart_before_buttons', 'display_mini_cart_total_weight' );