fix(eval): treat empty diff as conservative fallback in auto-close gate

Ganymede review nit: if get_pr_diff returns an empty string (edge case —
Forgejo quirk, empty PR), the old `if diff is None` branch would miss it,
the `elif diff and ...` would evaluate False (empty string is falsy), and
control would fall to `else` — triggering auto-close on zero diff content.

Change `if diff is None` → `if not diff` so empty string ALSO falls through
to the conservative path. Matches the stated posture: skip auto-close when
in doubt.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/eval_actions.py

@@ -167,10 +167,10 @@ async def dispose_rejected_pr(conn, pr_number: int, eval_attempts: int, all_issu
                 )
                 diff = None
 
-            if diff is None:
-                # Conservative: fall through to attempt-count branches below
+            if not diff:
+                # None or empty — conservative fallback, fall through to attempt-count branches
                 pass
-            elif diff and "new file mode" not in diff:
+            elif "new file mode" not in diff:
                 logger.info(
                     "PR #%d: near_duplicate but modifies-only (enrichment) — skipping auto-close",
                     pr_number,