MySQLdump with a blank password - syntax question

I'm running a mysqldump on a local machine, and the database has a blank password (not my call, and for whatever reason it needs to say blank).

I'm running the following:

mysqldump --opt -Q -u username -p databasename > output file

If I leave nothing after the -p (instead of -pPassword where an actual pw would be) it prompts for a password when run from a shell script.

What's the proper syntax to dump a mysql database from the command line for a database with a blank password? Any insight would be appreciated.