eye inconsistency with NumPy for dtype=None

[tomwhite]

What happened:

Calling eye with dtype=None gives an error rather than using the default float dtype, which is what happens in NumPy.

What you expected to happen:

Behaviour the same as NumPy.

Minimal Complete Verifiable Example:

>>> import numpy as np
>>> np.eye(2, dtype=None)
array([[1., 0.],
       [0., 1.]])
>>> import dask.array as da
>>> da.eye(2, dtype=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tom/projects-workspace/dask/dask/array/creation.py", line 546, in eye
    vchunks, hchunks = normalize_chunks(chunks, shape=(N, M), dtype=dtype)
  File "/Users/tom/projects-workspace/dask/dask/array/core.py", line 2907, in normalize_chunks
    chunks = auto_chunks(chunks, shape, limit, dtype, previous_chunks)
  File "/Users/tom/projects-workspace/dask/dask/array/core.py", line 3000, in auto_chunks
    raise TypeError("dtype must be known for auto-chunking")
TypeError: dtype must be known for auto-chunking

Anything else we need to know?:

This fix is needed for the Array API (dask/community#109).

Environment:

• Dask version: main
• Python version: 3.8
• Operating System: Mac
• Install method (conda, pip, source): source