bugfix: items.Array __init__ didn't ignore Null that _group_values added.

[laixintao]

The issue is, #218 added a _group_values cloud add Null into the Array values, but the Array's list.__init__ didn't ignore this value.

Poetry will copy.copy an Array item (for e.g. here: https://github.com/python-poetry/poetry-core/blob/main/src/poetry/core/packages/package.py#L290 ), then there will be an extra Null object in the values, so that an Array like this:

[tool.poetry]
classifiers = [
# comment
    "a",
    "b",
]

originally is ["a", "b"], after being copy.copyed, will become [None, "a", "b"] due to the Null. (Please see the testcase I added)

Then it will cause poetry to fail because there is an extra "None" in classifiers. ( https://github.com/laixintao/iredis/runs/7749027851?check_suite_focus=true )

related: #218

[laixintao]

I found this issue in iredis' build:

It coincidentally has a comment line in classifiers:
https://github.com/laixintao/iredis/blob/d0cafb62607868fa9859214d1cefe9ae29d6b273/pyproject.toml#L12

the poetry failure: https://github.com/laixintao/iredis/runs/7749027851?check_suite_focus=true

cc @frostming

[frostming]

Thanks for catching this