password column needs to allow null (oc7 backport)

password column needs to allow null otherwise Oracle will break for empty passwords

This problem was discovered here: https://github.com/owncloud/core/pull/12567#issuecomment-65516071

Oracle treats empty strings as null so we need to allow null for the password field otherwise non-password protected server-to-server shares will fail on Oracle.

cc @icewind1991 

@karlitschek do you agree on the backport?