avoid exception when __frame_data["code_context"] is None (scientific-python#83)

Author: dschult

lazy_loader/__init__.py

@@ -112,7 +112,7 @@ def __getattr__(self, x):
  f"No module named '{fd['spec']}'\n\n"
  "This error is lazily reported, having originally occured in\n"
  f' File {fd["filename"]}, line {fd["lineno"]}, in {fd["function"]}\n\n'
- f'----> {"".join(fd["code_context"]).strip()}'
+ f'----> {"".join(fd["code_context"] or "").strip()}'
  )