Fix PHP Error Notice if `@return []` found

- While PHPCompleteExtendedGenerateIndex is running, if encounters a
  method with `@return []` in the docblock it will issue a error:
  "PHP Notice: Uninitialized string offset: 0 in
  .vim/bundle/phpcomplete-extended/bin/IndexGenerator.php
  on line 914" because '[' and ']' are trim'ed off on a previous line.