Add OnDamaged method to IOnDamageBehavior (#685)

* Add OnDamaged method to IOnDamageBehavior

* Adds Source, SourceMob to OnDamage.

Content.Server/Interfaces/GameObjects/IOnDamageBehavior.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using Content.Server.GameObjects;
 
 namespace Content.Server.Interfaces
@@ -15,13 +16,20 @@ namespace Content.Server.Interfaces
         /// Gets a list of all DamageThresholds this component/entity are interested in.
         /// </summary>
         /// <returns>List of DamageThresholds to be added to DamageableComponent for watching.</returns>
-        List<DamageThreshold> GetAllDamageThresholds();
+        List<DamageThreshold> GetAllDamageThresholds() => null;
 
         /// <summary>
         /// Damage threshold passed event hookup.
         /// </summary>
         /// <param name="obj">Damageable component.</param>
         /// <param name="e">Damage threshold and whether it's passed in one way or another.</param>
-        void OnDamageThresholdPassed(object obj, DamageThresholdPassedEventArgs e);
+        void OnDamageThresholdPassed(object obj, DamageThresholdPassedEventArgs e) { }
+
+        /// <summary>
+        /// Called when the entity is damaged.
+        /// </summary>
+        /// <param name="obj">Damageable component.</param>
+        /// <param name="e">DamageEventArgs</param>
+        void OnDamaged(object obj, DamageEventArgs e) { }
     }
 }