Fix .isDiskPresent() always reporting true

We were still determining if the stack was null, but post-1.11 this will
never be the case.

src/main/java/dan200/computercraft/shared/peripheral/diskdrive/DiskDrivePeripheral.java

@@ -62,9 +62,9 @@ public class DiskDrivePeripheral implements IPeripheral
         {
             case 0:
             {
-                // isPresent
+                // isDiskPresent
                 return new Object[] {
-                    m_diskDrive.getDiskStack() != null
+                    !m_diskDrive.getDiskStack().isEmpty()
                 };
             }
             case 1:

src/main/java/dan200/computercraft/shared/peripheral/diskdrive/TileDiskDrive.java

@@ -423,6 +423,7 @@ public class TileDiskDrive extends TilePeripheralBase
         return new DiskDrivePeripheral( this );
     }
 
+    @Nonnull
     public ItemStack getDiskStack()
     {
         synchronized( this )