Fix possible NPE in 1.17->1.16.4 bedrock at 0 emulation (#821)

common/src/main/java/com/viaversion/viabackwards/protocol/v1_17to1_16_4/rewriter/BlockItemPacketRewriter1_17.java

@@ -364,6 +364,7 @@ public final class BlockItemPacketRewriter1_17 extends BackwardsItemRewriter<Cli
 
             if (ViaBackwards.getConfig().bedrockAtY0()) {
                 final ChunkSection lowestSection = chunk.getSections()[0];
+                if (lowestSection != null) {
                     final DataPalette blocks = lowestSection.palette(PaletteType.BLOCKS);
                     for (int x = 0; x < 16; x++) {
                         for (int z = 0; z < 16; z++) {
@@ -371,6 +372,7 @@ public final class BlockItemPacketRewriter1_17 extends BackwardsItemRewriter<Cli
                         }
                     }
                 }
+            }
 
             chunk.getBlockEntities().removeIf(compound -> {
                 NumberTag tag = compound.getNumberTag("y");