chore(frontend-python): don't require coverage for highlighting result

frontends/concrete-python/concrete/fhe/representation/graph.py

@@ -372,7 +372,7 @@ def format(
         for node in self.ordered_outputs():
             returns.append(f"%{id_map[node]}")
         lines.append(f"return {', '.join(returns)}")
-        if highlighted_result:
+        if highlighted_result:  # pragma: no cover
             highlighted_lines[len(lines) - 1] = highlighted_result
 
         # strip whitespaces