Fix the module to correctly list mailbox with empty name (e.g. " ")

Currently, the module parse LIST/XLIST responses from server incorrectly when there is a mailbox with the name " " (one space). I've fixed the regexp to allow parsing of such responses.

I've also allowed empty string in mailbox path components. But why did you filter out empty strings from path components in the first place? Is there some point I am missing here?