Fix NEI search always showing void cell (#300)

src/main/java/com/glodblock/github/common/storage/FluidCellInventory.java

@@ -13,6 +13,8 @@ import net.minecraft.nbt.NBTBase;
 import net.minecraft.nbt.NBTTagCompound;
 import net.minecraftforge.fluids.FluidStack;
 
+import org.jetbrains.annotations.NotNull;
+
 import com.glodblock.github.common.item.ItemBaseInfinityStorageCell;
 import com.glodblock.github.common.item.ItemFluidVoidStorageCell;
 import com.glodblock.github.crossmod.extracells.storage.ProxyFluidCellInventory;
@@ -476,7 +478,7 @@ public class FluidCellInventory implements IFluidCellInventory {
     }
 
     @Override
-    public List<IAEFluidStack> getContents() {
+    public @NotNull List<IAEFluidStack> getContents() {
         List<IAEFluidStack> ret = new ArrayList<>();
         for (IAEFluidStack fluid : this.getCellFluids()) {
             ret.add(fluid);

src/main/java/com/glodblock/github/common/storage/FluidVoidStorageCellInventory.java

@@ -1,11 +1,14 @@
 package com.glodblock.github.common.storage;
 
+import java.util.Collections;
 import java.util.List;
 
 import javax.annotation.Nonnull;
 
 import net.minecraft.item.ItemStack;
 
+import org.jetbrains.annotations.NotNull;
+
 import appeng.api.config.Actionable;
 import appeng.api.exceptions.AppEngException;
 import appeng.api.networking.security.BaseActionSource;
@@ -109,7 +112,7 @@ public class FluidVoidStorageCellInventory extends FluidCellInventory {
     }
 
     @Override
-    public List<IAEFluidStack> getContents() {
-        return null;
+    public @NotNull List<IAEFluidStack> getContents() {
+        return Collections.emptyList();
     }
 }

src/main/java/com/glodblock/github/common/storage/IFluidCellInventory.java

@@ -5,6 +5,8 @@ import java.util.List;
 import net.minecraft.inventory.IInventory;
 import net.minecraft.item.ItemStack;
 
+import org.jetbrains.annotations.NotNull;
+
 import appeng.api.storage.IMEInventory;
 import appeng.api.storage.data.IAEFluidStack;
 
@@ -90,6 +92,7 @@ public interface IFluidCellInventory extends IMEInventory<IAEFluidStack> {
      */
     long getTotalFluidTypes();
 
+    @NotNull
     List<IAEFluidStack> getContents();
 
     /**