How to have a tca field of type float/double but nullable

Double2With4DecimalsFormat.php

<?php
class Double2With4DecimalsFormat {
	function returnFieldJS() {
		return '
			return value.replace(",", ".");
		';
	}
	function evaluateFieldValue($value, $is_in, &$set) {
		if ('' == $value) {
			return NULL;
		}
		return sprintf('%01.4f', $value);
	}
}

ext_localconf.php

<?php
$TYPO3_CONF_VARS['SC_OPTIONS']['tce']['formevals']['Double2With4DecimalsFormat'] = 'EXT:nmidbproducts/Classes/Formevals/Double2With4DecimalsFormat.php';

ext_tables.sql

index_value double(11,4) DEFAULT NULL,

SomeTCA.php

<?php
'index_value' => array(
	'exclude' => 1,
	'label' => 'LLL:EXT:nmidbproducts/Resources/Private/Language/locallang_db.xlf:tx_nmidbproducts_domain_model_fructose.index_value',
	'config' => array(
		'type' => 'input',
		'size' => 30,
		'eval' => 'Double2With4DecimalsFormat'
	)
),