Fix mypy type narrowing error in cfg.py by adding None check before identity comparison

Agent-Logs-Url: https://github.com/MatthieuDartiailh/bytecode/sessions/b7904bd0-bbc8-412f-8f14-de2ed29696a1

Co-authored-by: MatthieuDartiailh <4481740+MatthieuDartiailh@users.noreply.github.com>

Author: Copilot

src/bytecode/cfg.py

@@ -363,7 +363,7 @@ def run(self) -> Generator[Union["_StackSizeComputer", int], int, None]:
                     # through a particular jump. So we are lenient here.
                     if (
                         te := self.block.get_trailing_try_end(i)
-                    ) and te.entry is self._current_try_begin:
+                    ) and self._current_try_begin is not None and te.entry is self._current_try_begin:
                         assert isinstance(te.entry.target, BasicBlock)
                         yield from self._compute_exception_handler_stack_usage(
                             te.entry.target,