Merge pull request #1806 from Icinga/bugfix/render-empty-strings

ConfigHelper: Ensure empty vars are not written as void to config

library/Director/IcingaConfig/IcingaConfigHelper.php

@@ -406,6 +406,10 @@ class IcingaConfigHelper
             $parts[] = static::renderString(substr($string, $offset, $i - $offset));
         }
 
-        return implode(' + ', $parts);
+        if (! empty($parts)) {
+            return implode(' + ', $parts);
+        } else {
+            return '""';
+        }
     }
 }