Setting website config to blank, Magento uses default

Question

Magento doesn't seem to like me setting blank config values at website or store level.

Steps to reproduce (1.6.2.0, 1.7.0.1, 1.7.0.2)

1) change config scope to website
2) untick use default next to 'default welcome message'
3) clear the text in the field
4) save

At this stage Magento will re-tick the use default box, however, it has saved my blank value. The next time I save this config page the config value from the default level is used.

This functionality was not present in 1.4, and I don't have 1.5 to test.

My question is, how do I create blank config values at the website or store level?

---

Update

In version 1.7.02 the code looks like this:

            $path = (string)$element->config_path;
            if (empty($path)) {
                $path = $section->getName() . '/' . $group->getName() . '/' . $fieldPrefix . $element->getName();
            } elseif (strrpos($path, '/') > 0) {
                // Extend config data with new section group
                $groupPath = substr($path, 0, strrpos($path, '/'));
                if (!isset($configDataAdditionalGroups[$groupPath])) {
                    $this->_configData = $this->_configDataObject->extendConfig(
                        $groupPath,
                        false,
                        $this->_configData
                    );
                    $configDataAdditionalGroups[$groupPath] = true;
                }
            }

And it uses empty() to evaluate the string.

Does anyone have a suggestion to modify this code to allow empty strings that I specify to override the default?

Answer 1

The problem comes from here:
Mage_Adminhtml_Block_System_Config_Form::initFields()

[For Magento 1.7.0.0]
Around line 295 there is this piece of code:

if (isset($this->_configData[$path])) {
    $data = $this->_configData[$path];
    $inherit = false;
} else {
    $data = $this->_configRoot->descend($path);
    $inherit = true;
}

This determines if the 'Use website/Use default value' checkbox is checked or not. The problem is that when a value is empty $this->_configData[$path] is null. And the function isset() returns true only if the parameter exists and is not null.
If you replace
if (isset($this->_configData[$path])) {
with
if (array_key_exists($path, $this->_configData)) {
it should work.

[For Magento 1.7.0.2]
In 1.7.0.2 version the code from above was moved in Mage_Adminhtml_Model_Config_Data::getConfigDataValue()

It's a similar piece of code and the solution is the same:
At line 388 there is this:

if (isset($configData[$path])) {

that should be replaced by this:

if (array_key_exists($path, $configData)) {