The default preParse() method - FTPFileEntryParserImpl.preParse() - parses every entry, and removes any that do not parse.
The entries are later parsed again to produce the main output.
It would be more efficient to weed out null entries once after parsing.
The preParse() method should be reserved for cases where it is strictly necessary, e.g. VMSVersioningFTPEntryParser and MVSFTPEntryParser which already override the default implementation.