Handle empty string case in maybe_trim_space

[aronson]

I ran into this issue when testing a locally-generated model based on Qwen.

----------------------------------------
Exception occurred during processing of request from ('[IP REDACTED]', 32822)
Traceback (most recent call last):
  File "/opt/homebrew/Cellar/python@3.12/3.12.7_1/Frameworks/Python.framework/Versions/3.12/lib/python3.12/socketserver.py", line 318, in _handle_request_noblock
    self.process_request(request, client_address)
  File "/opt/homebrew/Cellar/python@3.12/3.12.7_1/Frameworks/Python.framework/Versions/3.12/lib/python3.12/socketserver.py", line 349, in process_request
    self.finish_request(request, client_address)
  File "/opt/homebrew/Cellar/python@3.12/3.12.7_1/Frameworks/Python.framework/Versions/3.12/lib/python3.12/socketserver.py", line 362, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/Users/isaac/venv/lib/python3.12/site-packages/mlx_lm/server.py", line 758, in <lambda>
    lambda *args, **kwargs: handler_class(
                            ^^^^^^^^^^^^^^
  File "/Users/isaac/venv/lib/python3.12/site-packages/mlx_lm/server.py", line 200, in __init__
    super().__init__(*args, **kwargs)
  File "/opt/homebrew/Cellar/python@3.12/3.12.7_1/Frameworks/Python.framework/Versions/3.12/lib/python3.12/socketserver.py", line 761, in __init__
    self.handle()
  File "/opt/homebrew/Cellar/python@3.12/3.12.7_1/Frameworks/Python.framework/Versions/3.12/lib/python3.12/http/server.py", line 436, in handle
    self.handle_one_request()
  File "/opt/homebrew/Cellar/python@3.12/3.12.7_1/Frameworks/Python.framework/Versions/3.12/lib/python3.12/http/server.py", line 424, in handle_one_request
    method()
  File "/Users/isaac/venv/lib/python3.12/site-packages/mlx_lm/server.py", line 313, in do_POST
    method(prompt, stop_id_sequences)
  File "/Users/isaac/venv/lib/python3.12/site-packages/mlx_lm/server.py", line 594, in handle_stream
    detokenizer.add_token(token)
  File "/Users/isaac/venv/lib/python3.12/site-packages/mlx_lm/tokenizer_utils.py", line 210, in add_token
    self.text += self._maybe_trim_space(current_text)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/isaac/venv/lib/python3.12/site-packages/mlx_lm/tokenizer_utils.py", line 196, in _maybe_trim_space
    if current_text[0] != " ":
       ~~~~~~~~~~~~^^^
IndexError: string index out of range
----------------------------------------

This covers a gap in the logic and should help future-proof the function's use.

[chan4lk]

Tested this on my local machine (M3 Pro). And working well

[awni]

If the current_text is ever None then taking the len will raise an exception, so I don't think it makes sense to check for that.

[aronson]

Thanks, makes sense and the if condition improvement you made looks good to me.

[awni]

Thanks for the fix! Will merge as soon as tests clear.