issue_comments: 629346101

Okay so I've traced the error back to the map_blocks() function. I don't fully understand the code for this function in xarray/core/parallel.py, but here's a quick report on the different behaviours.

Normally, when using the make_ds() and make_da() functions in test_dask.py, without any changes to ds.chunk() we have:

```python

def func(obj): ... result = obj + obj.x + 5 * obj.y ... return result ... xr.map_blocks(func, ds).unify_chunks().chunks Frozen(SortedKeysDict({'x': (4, 4, 2), 'y': (5, 5, 5, 5), 'z': (4,)})) func(ds).chunk().unify_chunks().chunks Frozen(SortedKeysDict({'x': (4, 4, 2), 'y': (5, 5, 5, 5), 'z': (4,)})) ```

However, when I use the changes I've made to dataset.py (changing isinstance(chunks, Number) to is_scalar(chunks)), the behaviour becomes:

```python

xr.map_blocks(func, ds).unify_chunks().chunks Frozen(SortedKeysDict({'x': (4, 4, 2), 'y': (5, 5, 5, 5), 'z': (4,)})) func(ds).chunk().unify_chunks().chunks Frozen(SortedKeysDict({'x': (10,), 'y': (20,), 'z': (4,)})) ```

Which means that it now fails the test_map_blocks() call in test_dask.py line 1077.

I've tried to follow through the code and see what is actually happening when this change is made, but I'm out of my depth here. My guess is that is_scalar(chunks) is giving the wrong behaviour when chunks=None ?

Edit: I think that's the problem!
```python

isinstance(None, numbers.Number) False is_scalar(None) True ```

I'll add in something to catch Nones and see if it fixes the error...