perf: trivial changes

[radoering]

Not much gain, but trivial changes that do not affect maintainability.

1. We already cache parse_constraint (some lines above) so that it just makes sense to also cache parse_marker_version_constraint.
2. We can avoid the creation of a throwaway empty list.

[sourcery-ai]

Hey - I've left some high level feedback:

• Using functools.cache makes the cache unbounded; consider functools.lru_cache with a reasonable maxsize (as is likely done for parse_constraint) to avoid unbounded memory growth in long‑running processes.
• If the project still supports Python versions prior to 3.9, double-check that functools.cache is available in the minimum supported version or align with whatever decorator is used to cache parse_constraint for consistency.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Using `functools.cache` makes the cache unbounded; consider `functools.lru_cache` with a reasonable `maxsize` (as is likely done for `parse_constraint`) to avoid unbounded memory growth in long‑running processes.
- If the project still supports Python versions prior to 3.9, double-check that `functools.cache` is available in the minimum supported version or align with whatever decorator is used to cache `parse_constraint` for consistency.

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