Fix fill_value handling for complex dtypes

[dcherian]

Broken out of #2189

1. Property test now tests arbitrary fill_values of the right dtype, as well as None
2. All handle v2, v3 dtypes

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[dcherian]

FWIW the zarr.json is

{
  "shape": [
    1
  ],
  "fill_value": 0,
  "chunk_grid": {
    "name": "regular",
    "configuration": {
      "chunk_shape": [
        1
      ]
    }
  },
  "attributes": {},
  "zarr_format": 3,
  "data_type": "datetime64[Y]",
  "chunk_key_encoding": {
    "name": "default",
    "configuration": {
      "separator": "/"
    }
  },
  "codecs": [
    {
      "name": "bytes",
      "configuration": {
        "endian": "little"
      }
    }
  ],
  "node_type": "array"
}

[dcherian]

also needed for V2

[dcherian]

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