Fluid rework derp fixes. (#6098)

* Fix nulls returning from non-null methods, and null-check of non-nullable values.
* Fix missed computation of the isEmpty flag in FluidStack
* Cleanup getAmount/isEmpty calls.
* Remove redundant suppliers. That info is readily available from the vanilla Fluid / FluidState.

src/main/java/net/minecraftforge/fluids/FluidStack.java

@@ -69,6 +69,8 @@ public class FluidStack
         }
         this.fluidDelegate = fluid.delegate;
         this.amount = amount;
+
+        updateEmpty();
     }
 
     public FluidStack(Fluid fluid, int amount, CompoundNBT nbt)
@@ -170,7 +172,6 @@ public class FluidStack
     public void setAmount(int amount)
     {
         if (getRawFluid() == Fluids.EMPTY) throw new IllegalStateException("Can't modify the empty stack.");
-        if (amount < 0) throw new IllegalStateException("The amount can't be negative.");
         this.amount = amount;
         updateEmpty();
     }
@@ -217,8 +218,10 @@ public class FluidStack
     {
         getOrCreateTag();
         CompoundNBT child = tag.getCompound(childName);
-        if (child == null)
-            tag.put(childName, child = new CompoundNBT());
+        if (!tag.contains(childName, Constants.NBT.TAG_COMPOUND))
+        {
+            tag.put(childName, child);
+        }
         return child;
     }
 
@@ -302,13 +305,8 @@ public class FluidStack
      *            The ItemStack for comparison
      * @return true if the Fluids (IDs and NBT Tags) are the same
      */
-    public boolean isFluidEqual(ItemStack other)
+    public boolean isFluidEqual(@Nonnull ItemStack other)
     {
-        if (other == null)
-        {
-            return false;
-        }
-
         return FluidUtil.getFluidContained(other).map(this::isFluidEqual).orElse(false);
     }