Fixed codec for v2 data with no fill value (#2207)

xref https://github.com/zarr-developers/zarr-python/pull/2179/files#r1765600769

Author: TomAugspurger

src/zarr/core/array.py

@@ -313,7 +313,7 @@ async def _create_v2(
             chunks=chunks,
             order=order,
             dimension_separator=dimension_separator,
-            fill_value=fill_value,
+            fill_value=0 if fill_value is None else fill_value,
             compressor=compressor,
             filters=filters,
             attributes=attributes,