Check if Dask is installed (#62)

Fix bug so that an isinstance check doesn't require dask to be installed to work.

---------

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>
Co-authored-by: Justus Magin <keewis@users.noreply.github.com>

cupy_xarray/accessors.py

@@ -1,3 +1,5 @@
+from typing import TYPE_CHECKING, Any
+
 import cupy as cp
 from xarray import (
     DataArray,
@@ -6,12 +8,16 @@
     register_dataset_accessor,
 )
 
+if TYPE_CHECKING:
+    DuckArrayTypes = tuple[type[Any], ...]
+    dask_array_type: DuckArrayTypes
+
 try:
     import dask.array
 
-    dask_array_type = dask.array.Array
+    dask_array_type = (dask.array.Array,)
 except ImportError:
-    dask_array_type = None
+    dask_array_type = ()
 
 
 @register_dataarray_accessor("cupy")