Use nullcontext in nl_decoder_engine when timer_manager.current is None

[mgoin]

There is an error that seems to only happen when generating the second token when streaming=True, but only shows up in specific pipeline setups such as Gradio

Error:

  File "/home/user/.local/lib/python3.10/site-packages/deepsparse/transformers/engines/nl_decoder_engine.py", line 194, in __call__
    with timer.time(f"EXECUTE_ENGINE_SEQ_LEN_{self.sequence_length}"):
AttributeError: 'NoneType' object has no attribute 'time'

This PR simply uses nullcontext to replace the timer if it is None

[mgoin]

Resolved by #1294