remove decalset if it has no decals

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Decal/DecalSystem.cs

                 }
             }
 
+            public int Count
+            {
+                get
+                {
+                    return this.m_DecalsCount;
+                }
+            }
+
             private CullingGroup m_CullingGroup = null;
             private BoundingSphere[] m_BoundingSpheres = new BoundingSphere[kDecalBlockSize];
             private DecalProjectorComponent[] m_Decals = new DecalProjectorComponent[kDecalBlockSize];
             if (m_DecalSets.TryGetValue(key, out decalSet))
             {
                 decalSet.RemoveDecal(decal);
+                if (decalSet.Count == 0)
+                {
+                    m_DecalSets.Remove(key);
+                }
             }
         }