Revert to original MergingShardBuilder (if/else not needed with faster buffer)

Author: balbasty

src/zarr/codecs/sharding.py

@@ -202,7 +202,7 @@ def create_empty(
             buffer_prototype = default_buffer_prototype()
         index = _ShardIndex.create_empty(chunks_per_shard)
         obj = cls()
-        obj.buf = buffer_prototype.buffer.create_zero_length()
+        obj.buf = buffer_prototype.buffer.Delayed.create_zero_length()
         obj.index = index
         return obj
 
@@ -585,16 +585,15 @@ async def _encode_partial_single(
         chunks_per_shard = self._get_chunks_per_shard(shard_spec)
         chunk_spec = self._get_chunk_spec(shard_spec)
 
-        shard_read = await self._load_full_shard_maybe(
-            byte_getter=byte_setter,
-            prototype=chunk_spec.prototype,
-            chunks_per_shard=chunks_per_shard,
+        shard_dict = _MergingShardBuilder(
+            await self._load_full_shard_maybe(
+                byte_getter=byte_setter,
+                prototype=chunk_spec.prototype,
+                chunks_per_shard=chunks_per_shard,
+            )
+            or _ShardReader.create_empty(chunks_per_shard),
+            _ShardBuilder.create_empty(chunks_per_shard),
         )
-        shard_build = _ShardBuilder.create_empty(chunks_per_shard)
-        if shard_read:
-            shard_dict = _MergingShardBuilder(shard_read, shard_build)
-        else:
-            shard_dict = shard_build
 
         indexer = list(
             get_indexer(