A lone carriage return between BB quote tags produces an empty, misshapen quote box (screenshot attached):
[quote]
[/quote]
Note that any other non-character quotation is very appropriately removed by the editor when you preview/submit the post, or when you switch to source mode. So none of the following will generate the empty quote box:
[quote] [/quote]
[quote][i][/i][/quote]
[quote]COLOR="#FF0000"[/COLOR][/quote]