Handle more obscure display names.
Handle more obscure display names.

The missing From: header was properly handled before already, but now
the empty string as the display name and whitespace-only display names
are also catched. Examples:

    From: " " <only@whitespace.example>
    From: "" <emtpy@string.example>

Quintus/chessboard,Quintus/chessboard,Quintus/chessboard,Quintus/chessboard