fix cauterization (#23264)

* fix cauterization

* remove if statement

* revert DamageTest change

* fix

Content.Shared/Damage/DamageModifierSet.cs

@@ -1,4 +1,4 @@
-using Content.Shared.Damage.Prototypes;
+using Content.Shared.Damage.Prototypes;
 using Robust.Shared.Serialization;
 using Robust.Shared.Serialization.TypeSerializers.Implementations.Custom.Prototype.Dictionary;
 
@@ -12,7 +12,6 @@ namespace Content.Shared.Damage
     /// </summary>
     /// <remarks>
     /// The modifier will only ever be applied to damage that is being dealt. Healing is unmodified.
-    /// The modifier can also never convert damage into healing.
     /// </remarks>
     [DataDefinition]
     [Serializable, NetSerializable]

Content.Shared/Damage/DamageSpecifier.cs

@@ -151,12 +151,12 @@ namespace Content.Shared.Damage
                 float newValue = value.Float();
 
                 if (modifierSet.FlatReduction.TryGetValue(key, out var reduction))
-                    newValue -= reduction;
+                    newValue = Math.Max(0f, newValue - reduction); // flat reductions can't heal you
 
                 if (modifierSet.Coefficients.TryGetValue(key, out var coefficient))
-                    newValue *= coefficient;
+                    newValue *= coefficient; // coefficients can heal you, e.g. cauterizing bleeding
 
-                if (newValue > 0)
+                if(newValue != 0)
                     newDamage.DamageDict[key] = FixedPoint2.New(newValue);
             }