Change drop_axis to default to None rather than an empty list in …

…`map_blocks`. (#446)

This follows Dask, see dask/dask#9462.

cubed/core/ops.py

@@ -564,13 +564,16 @@ def map_blocks(
     *args: "Array",
     dtype=None,
     chunks=None,
-    drop_axis=[],
+    drop_axis=None,
     new_axis=None,
     spec=None,
     **kwargs,
 ) -> "Array":
     """Apply a function to corresponding blocks from multiple input arrays."""
 
+    if drop_axis is None:
+        drop_axis = []
+
     # Handle the case where an array is created by calling `map_blocks` with no input arrays
     if len(args) == 0:
         from cubed.array_api.creation_functions import empty_virtual_array
@@ -618,10 +621,19 @@ def wrap(*a, **kw):
 
 
 def _map_blocks(
-    func, *args: "Array", dtype=None, chunks=None, drop_axis=[], new_axis=None, **kwargs
+    func,
+    *args: "Array",
+    dtype=None,
+    chunks=None,
+    drop_axis=None,
+    new_axis=None,
+    **kwargs,
 ) -> "Array":
     # based on dask
 
+    if drop_axis is None:
+        drop_axis = []
+
     new_axes = {}
 
     if isinstance(drop_axis, Number):