Fix get_metadata() function response, NIL response was introducing an shift

This is a fix for get_metadata() function. Varibles are not incremented correctly in rcube_imap_generic.php when there is a NIL response to GETMETADATA  IMAP command. It is resulting in shift in get_metadata() function answer.:
example:
  'Shared Folders/bnogas/kalendarz2' => 
  array (
    '' => '/shared/vendor/kolab/folder-type',
  ),
instead of:
  'Shared Folders/bnogas/kalendarz2' => 
  array (
    '/shared/vendor/kolab/folder-type' => 'event',
  ),