Fix error on draft or new messages

[sumaiyamannan]

Hi
This fixes the below error which is thrown in two scenarios:

1. If the message field is left blank for new messages;
2. When viewing a draft message.

The error is:
"Exception - Too few arguments to function mod_dialogue_conversation_form::{closure}(), 1 passed in [dirroot]/lib/form/autocomplete.php on line 245 and exactly 3 expected"

Regards,
Sumaiya