`ChatDatabricks()` chokes on empty responses with Anthropic models

[cpsievert]

Related to #86

from chatlas import ChatDatabricks

chat = ChatDatabricks(model="databricks-claude-3-7-sonnet")
chat.chat("Respond with only two blank lines")
chat.chat("What's 1+1? Just give me the number")

Traceback (most recent call last):
  File "/Users/cpsievert/github/chatlas/sandbox/_databricks.py", line 5, in <module>
    resp = chat.chat("What's 1+1? Just give me the number")
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cpsievert/github/chatlas/chatlas/_chat.py", line 575, in chat
    for _ in response:
  File "/Users/cpsievert/github/chatlas/chatlas/_chat.py", line 1581, in __next__
    chunk = next(self._generator)
            ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cpsievert/github/chatlas/chatlas/_chat.py", line 1195, in _chat_impl
    for chunk in self._submit_turns(
  File "/Users/cpsievert/github/chatlas/chatlas/_chat.py", line 1305, in _submit_turns
    response = self.provider.chat_perform(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cpsievert/github/chatlas/chatlas/_openai.py", line 238, in chat_perform
    return self._client.chat.completions.create(**kwargs)  # type: ignore
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cpsievert/github/chatlas/.venv/lib/python3.11/site-packages/openai/_utils/_utils.py", line 287, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cpsievert/github/chatlas/.venv/lib/python3.11/site-packages/openai/resources/chat/completions/completions.py", line 925, in create
    return self._post(
           ^^^^^^^^^^^
  File "/Users/cpsievert/github/chatlas/.venv/lib/python3.11/site-packages/openai/_base_client.py", line 1239, in post
    return cast(ResponseT, self.request(cast_to, opts, stream=stream, stream_cls=stream_cls))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cpsievert/github/chatlas/.venv/lib/python3.11/site-packages/openai/_base_client.py", line 1034, in request
    raise self._make_status_error_from_response(err.response) from None
openai.BadRequestError: Error code: 400 - {'error_code': 'BAD_REQUEST', 'message': 'messages: text content blocks must contain non-whitespace text'}