Ignore PrepareBlockDropsTransaction when generating ChangeBlockEvent.All

The transaction is used to associate the drops to block transaction
but it does not actually contain any relevant information about
the block change

src/main/java/org/spongepowered/common/event/tracking/context/transaction/block/BlockEventBasedTransaction.java

@@ -77,6 +77,9 @@ public final Optional<ChangeBlockEvent.All> generateEvent(
         final Map<BlockPos, BlockTransaction> eventTransactions = new HashMap<>();
         for (final GameTransaction<@NonNull ?> transaction : transactions) {
             final BlockEventBasedTransaction blockTransaction = (BlockEventBasedTransaction) transaction;
+            if (!blockTransaction.actualBlockTransaction()) {
+                continue;
+            }
             final SpongeBlockSnapshot original = blockTransaction.getOriginalSnapshot();
             final SpongeBlockSnapshot result = blockTransaction.getResultingSnapshot();
             final Operation operation = context.getBlockOperation(original, result);
@@ -101,6 +104,10 @@ public final Optional<ChangeBlockEvent.All> generateEvent(
         ));
     }
 
+    protected boolean actualBlockTransaction() {
+        return true;
+    }
+
     protected abstract SpongeBlockSnapshot getResultingSnapshot();
 
     protected abstract SpongeBlockSnapshot getOriginalSnapshot();

src/main/java/org/spongepowered/common/event/tracking/context/transaction/block/PrepareBlockDropsTransaction.java

@@ -52,9 +52,14 @@ public PrepareBlockDropsTransaction(
         this.originalState = original;
     }
 
+    @Override
+    protected boolean actualBlockTransaction() {
+        return false;
+    }
+
     @Override
     protected SpongeBlockSnapshot getResultingSnapshot() {
-        return null;
+        throw new UnsupportedOperationException();
     }
 
     @Override