Fix for TypeError in llm templates list when template contains empty prompt

[sherwind]

A TypeError is getting triggered when running the command llm templates list if a template contains an empty prompt.

To reproduce the issue, quickly close the editor after using the llm templates edit new-prompt-name command.

For reference, here's a snippet of the traceback:

  File ".../lib/python3.9/site-packages/llm/cli.py", line 545, in templates_list
    pairs.append((name, "".join(text).replace("\n", " ")))
TypeError: sequence item 0: expected str instance, NoneType found

[simonw]

Thanks, useful fix.