Do not send Attacked message on entities that have no attack effects receiver

Since rP22754, it is possible for units to be sent "Attacked" messages even though they cannot be attacked
(e.g. a non-capturable being the target of a Capture attack effect).
These messages are spurious and should not be sent.

Reported By: Angen

Differential Revision: https://code.wildfiregames.com/D2758