[BUGFIX:BP:11.2] Use plugin namespace as label for flexforms

[dkd-friedrich]

Backport of #3549

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When you configure additional pluginnamespaces via extension configuration, the dropdown in the plugin flexform shows Default followed by empty entries.

Extension Configuration to reproduce this:

$GLOBALS['TYPO3_CONF_VARS']['EXTENSIONS']['solr']['pluginNamespaces'] => 'search,sitesearch,newssearch,eventsearch'

The view.pluginNamespace DropDown shows up with the folowing Field:

<select class="form-select form-control-adapt">
    <option value="tx_solr" selected="selected">Default</option>
    <option value="search"></option>
    <option value="sitesearch"></option>
    <option value="newssearch"></option>
    <option value="eventsearch"></option>
</select>

It is hard to select an plugin namespace with empty labels.

The user function that returns the entries creates the labels with
$label = $namespace === 'tx_solr' ? 'Default' : '':

ext-solr/Classes/System/UserFunctions/FlexFormUserFunctions.php

Lines 189 to 198 in d095058

	public function getAvailablePluginNamespaces(array &$parentInformation)
	{
	$extensionConfiguration = GeneralUtility::makeInstance(ExtensionConfiguration::class);
	$namespaces = [];
	foreach ($extensionConfiguration->getAvailablePluginNamespaces() as $namespace) {
	$label = $namespace === 'tx_solr' ? 'Default' : '';
	$namespaces[$namespace] = [$label, $namespace];
	}
	$parentInformation['items'] = $namespaces;
	}

This pull requests uses the namespace instead of an empty string for the label:
$label = $namespace === 'tx_solr' ? 'Default' : $namespace