BlockStateUpgrader: fixed undefined array key when flattening a block…

… whose new ID is the same as the old

this came up during 1.20.50 testing, where minecraft:stone[stone_type=stone] continues to have an ID minecraft:stone without the stone_type property.
This could have appeared in other ways, such as with an invalid legacy blockstate.

src/data/bedrock/block/upgrade/BlockStateUpgrader.php

@@ -86,7 +86,7 @@ public function upgrade(BlockStateData $blockStateData) : BlockStateData{
 					if(is_string($remap->newName)){
 						$newName = $remap->newName;
 					}else{
-						$flattenedValue = $oldState[$remap->newName->flattenedProperty];
+						$flattenedValue = $oldState[$remap->newName->flattenedProperty] ?? null;
 						if($flattenedValue instanceof StringTag){
 							$newName = sprintf("%s%s%s", $remap->newName->prefix, $flattenedValue->getValue(), $remap->newName->suffix);
 							unset($oldState[$remap->newName->flattenedProperty]);