Adressed PR comments

src/main/java/com/refinedmods/refinedstorage/apiimpl/network/node/cover/CoverManager.java

@@ -164,8 +164,8 @@ public class CoverManager {
     }
 
     @Nullable
-    public static Block getBlock(@Nullable ItemStack item) {
-        if (item == null) {
+    public static Block getBlock(ItemStack item) {
+        if (item.isEmpty()) {
             return null;
         }
 
@@ -180,7 +180,7 @@ public class CoverManager {
 
     @Nullable
     @SuppressWarnings("deprecation")
-    public static BlockState getBlockState(@Nullable ItemStack item) {
+    public static BlockState getBlockState(ItemStack item) {
         Block block = getBlock(item);
 
         if (block == null) {

src/main/java/com/refinedmods/refinedstorage/render/model/CubeBuilder.java

@@ -111,7 +111,7 @@ public class CubeBuilder {
     }
 
     private BakedQuad bakeFace(Direction facing, Face cubeFace) {
-        BakedQuadBuilder builder = new BakedQuadBuilder(cubeFace.sprite); //TODO See if can change the vertex format
+        BakedQuadBuilder builder = new BakedQuadBuilder(cubeFace.sprite);
 
         builder.setQuadOrientation(facing);
         builder.setQuadTint(-1);

src/main/java/com/refinedmods/refinedstorage/util/RenderUtils.java

@@ -373,7 +373,9 @@ public final class RenderUtils {
         }
 
         if (sprite == null) {
-            sprite = null; //TODO Get missing sprite
+            for (BakedQuad quad : Minecraft.getInstance().getModelManager().getMissingModel().getQuads(coverState, facing, rand)) {
+                return quad.getSprite();
+            }
         }
 
         return sprite;