Fixes server crash due to null construction step

ConstructionComponent.AttackBy tries calling TryProcessStep on the forward step of the construction stage, and if that fails it tries the backwards step. I many construction prototypes the backwards step is null for all stages and so clicking the construction ghost with an invalid step results in a crash due to the step passed to TryProcessStep being null.

Content.Server/GameObjects/Components/Construction/ConstructionComponent.cs

@@ -93,6 +93,10 @@ namespace Content.Server.GameObjects.Components.Construction
 
         bool TryProcessStep(ConstructionStep step, IEntity slapped)
         {
+            if (step == null)
+            {
+                return false;
+            }
             var sound = IoCManager.Resolve<IEntitySystemManager>().GetEntitySystem<AudioSystem>();
 
             switch (step)