Tech-debt: Since-time mode: `since=None` can crash `get_issues()`, and API failures silently degrade to empty results

[miroslavpojer]

Found while reviewing PR #337 (Copilot comments on the pagination-safety refactor, since reverted from that PR as out of scope for #335).

Problem 1 — since=None can raise inside PyGithub

In DataMiner._get_issues, data.since is Optional[datetime] and gets passed straight through to Repository.get_issues(..., since=data.since). PyGithub's implementation asserts is_optional(since, datetime), i.e. isinstance(v, NotSetType) or isinstance(v, datetime) — explicitly passing None satisfies neither, so get_issues(since=None) raises AssertionError: None instead of falling back to its default. Today this is masked because the call goes through _safe_call's untyped wrapper, and is also practically unreachable in this exact spot (a GitHub release always has created_at) — but it's a latent crash risk, and the same pattern (attribute typed Optional[datetime], passed unconditionally as a kwarg) could easily recur elsewhere.

Fix: only pass since when it's not None (omit the kwarg so PyGithub's NotSet default applies), instead of suppressing the type checker.

Problem 2 — _safe_call failures silently produce incomplete results in since-time mode

_safe_call catches GithubException/network errors and returns None. In compare mode, a None result is treated as fatal (sys.exit(1)) — e.g. comparison is None. In since-time mode, several fetches (repo.get_pulls, repo.get_commits, repo.get_releases, repo.get_issues) instead do ... or [], so a transient API failure silently becomes "0 PRs found" / "0 commits found" / "treat this as the first release" and the run completes "successfully" with incomplete or wrong release notes.

Decide and apply consistently: should a _safe_call failure during since-time mining be fatal (matching compare mode), or is partial/degraded output acceptable with a loud warning? Whichever is chosen, apply it uniformly across all the since-time-mode fetches instead of the current ad hoc mix of "crash" (pre-existing, for some paths) and "silently continue" (introduced in #337 for others).

Update - see latest post from copilot review