Handle missing attributes key from metadata, and other fixes

[d-v-b]

• handle creating v3 metadata from a dict without an attributes key (attributes will be set to None in the resulting metadata document).
• handle null entries in dimension_names correctly at validation
• ensure that collection types become tuple during metadata serialization
• make a copy of the input dict in from_dict method of array v3 metadata, to avoid mutating the input dict
• allow creating ChunkKeyEncoding from dicts without configuration, because it's optional.
• add a test case that revealed the problems fixed by all of the above changes.

closes #2024

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)

[d-v-b]

thanks for looking over it @normanrz!