fix: handle empty pa11y stdout and forward stderr for JSON decode errors

Co-authored-by: aider (o3) <aider@aider.chat>

check_wcag_unique.py

@@ -39,9 +39,19 @@ def run_pa11y(url: str) -> List[dict]:
         print(f"[ERROR] pa11y failed with code {result.returncode} for {url}", file=sys.stderr)
         return []
 
+    # Handle cases where pa11y writes nothing (or only whitespace) to stdout.
+    if result.stdout is None or not result.stdout.strip():
+        if result.stderr:
+            # Forward pa11y’s stderr so you can inspect what happened.
+            print(result.stderr, file=sys.stderr)
+        print(f"[WARN] pa11y produced no JSON output for {url}", file=sys.stderr)
+        return []
+
     try:
         return json.loads(result.stdout)
     except json.JSONDecodeError:
+        # Show the raw (invalid) output to aid troubleshooting.
+        print(result.stdout, file=sys.stderr)
         print(f"[WARN] Could not parse pa11y JSON output for {url}", file=sys.stderr)
         return []