Rethink session metadata default populations

Resolving mypy errors in https://github.com/con/nwb2bids/pull/387 required a lot of null checks prior to accessing fields known implicitly to be set by the later operations

Would reduce duplication to just rethink the way the object and its field initializes - something like a default population but safe retrieval (dict-like)

Start by reviewing PR #387 and the session metadata object and field initialization discussed in the issue. Define a safer default-population or dict-like retrieval approach that removes repeated null checks while preserving mypy correctness; done means the affected metadata accesses no longer require redundant checks and the mypy errors are resolved.

Written by the indexing model from the issue text.