Don't write the default value to disk

This matches the previous behaviour and should not be changed silently.

Signed-off-by: Pablo Zmdl <pablo@nextcloud.com>

php/src/Data/ConfigurationManager.php

@@ -591,7 +591,6 @@ class ConfigurationManager
         $configValue = $this->get($configName, '');
         if ($envVariableOutput === false) {
             if ($configValue === '') {
-                $this->set($configName, $defaultValue);
                 return $defaultValue;
             }
             return $configValue;