Adjust for small change in signature of private xarray function _nested_combine (#779)

* adjust for small change in the signature of the private xarray function xarray.structure.combine._nested_combine

* release note

Author: TomNicholas

docs/releases.md

@@ -8,6 +8,10 @@
 
 ### Bug fixes
 
+- Adjust for minor upcoming change in private xarray API `xarray.structure.combine._nested_combine`.
+  ([#779](https://github.com/zarr-developers/VirtualiZarr/pull/779)).
+  By [Tom Nicholas](https://github.com/TomNicholas).
+
 ### Documentation
 
 ### Internal changes

virtualizarr/xarray.py

@@ -17,6 +17,7 @@
 import xarray.indexes
 from xarray import DataArray, Dataset, Index, combine_by_coords
 from xarray.backends.common import _find_absolute_paths
+from xarray.core import dtypes
 from xarray.core.types import NestedSequence
 from xarray.structure.combine import _infer_concat_order_from_positions, _nested_combine
 
@@ -261,6 +262,7 @@ def _open(path: str) -> xr.Dataset:
                 ids=ids,
                 join=join,
                 combine_attrs=combine_attrs,
+                fill_value=dtypes.NA,
             )
         elif combine == "by_coords":
             # Redo ordering from coordinates, ignoring how they were ordered