Manual typing fix

By definition `T_Chunks` and `T_Engine` could actually be `None`.

Author: DimitriPapadopoulos

xarray/backends/api.py

@@ -710,8 +710,8 @@ def open_dataset(
 def open_dataarray(
     filename_or_obj: str | os.PathLike[Any] | ReadBuffer | AbstractDataStore,
     *,
-    engine: T_Engine | None = None,
-    chunks: T_Chunks | None = None,
+    engine: T_Engine = None,
+    chunks: T_Chunks = None,
     cache: bool | None = None,
     decode_cf: bool | None = None,
     mask_and_scale: bool | None = None,
@@ -1394,7 +1394,7 @@ def open_mfdataset(
     | os.PathLike
     | ReadBuffer
     | NestedSequence[str | os.PathLike | ReadBuffer],
-    chunks: T_Chunks | None = None,
+    chunks: T_Chunks = None,
     concat_dim: (
         str
         | DataArray
@@ -1406,7 +1406,7 @@ def open_mfdataset(
     ) = None,
     compat: CompatOptions = "no_conflicts",
     preprocess: Callable[[Dataset], Dataset] | None = None,
-    engine: T_Engine | None = None,
+    engine: T_Engine = None,
     data_vars: Literal["all", "minimal", "different"] | list[str] = "all",
     coords="different",
     combine: Literal["by_coords", "nested"] = "by_coords",