Use Union[None, T] instead of Optional[T] in SSZ datastructures

[ericsson49]

Recent PR #5180 introduced Vector[Optional[boolean], PTC_SIZE]] in gloas fork-choice.md.

However, Optional is not supported as SSZ type, which breaks the generated gloas spec, e.g.

% uv run python -c "from eth_consensus_specs.test import context"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/alexvlasov/Work/GitHub/consensus-specs/tests/core/pyspec/eth_consensus_specs/test/context.py", line 35, in <module>
    from .helpers.specs import (
  File "/Users/alexvlasov/Work/GitHub/consensus-specs/tests/core/pyspec/eth_consensus_specs/test/helpers/specs.py", line 16, in <module>
    exec(
  File "<string>", line 1, in <module>
  File "/Users/alexvlasov/Work/GitHub/consensus-specs/tests/core/pyspec/eth_consensus_specs/gloas/__init__.py", line 1, in <module>
    from . import mainnet as spec  # noqa:F401
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/alexvlasov/Work/GitHub/consensus-specs/tests/core/pyspec/eth_consensus_specs/gloas/mainnet.py", line 1248, in <module>
    class Store(object):
  File "/Users/alexvlasov/Work/GitHub/consensus-specs/tests/core/pyspec/eth_consensus_specs/gloas/mainnet.py", line 1268, in Store
    payload_timeliness_vote: Dict[Root, Vector[Optional[boolean], PTC_SIZE]] = field(
                                        ~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/alexvlasov/Work/GitHub/consensus-specs/.venv/lib/python3.11/site-packages/remerkleable/complex.py", line 605, in __class_getitem__
    if element_view_cls.is_fixed_byte_length():
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.11/3.11.12_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/typing.py", line 1317, in __getattr__
    return getattr(self.__origin__, attr)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.11/3.11.12_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/typing.py", line 474, in __getattr__
    raise AttributeError(item)
AttributeError: is_fixed_byte_length

The PR fixes this by using Vector[Union[None, boolean], PTC_SIZE] instead.

Related to #5180

[jtraglia]

So SSZ unions are not actually used anywhere in the specs and we were debating whether or not we should remove them (#3906). Also, implementers of this would probably just use non-SSZ types here. Let me look into if switching away from SSZ types here, for this particular field. If it's not trivial, we can merge this PR and deal with it later.

[etan-status]

Store is not an SSZ container, what's the problem with optional?
If Optional is problematic, this should be PyUnion, not SSZ Union.

[jtraglia]

The problem was that it was used inside Vector which is an SSZ type. We shouldn't have been using Vector here in the first place. I should have caught this earlier. I made #5208 to fix this.

[jtraglia]

Closing this in favor of #5208.