AttributeError on custom_dictionary in tts.synthesize() since version 2.17

``Since version 2.17, an AttributeError occurs when using the tts.synthesize() method in the riva.client.tts module. It seems that the custom_dictionary parameter is None by default, which causes the function add_custom_dictionary_to_config to fail when attempting to access .items().

/home/rodrigo/Proyectos/marta/Call-Services/venv/bin/python /home/rodrigo/Proyectos/marta/Call-Services/test/performance_test/testNvidiaRivaTTS.py 
/home/rodrigo/Proyectos/marta/Call-Services/test/performance_test/testNvidiaRivaTTS.py:16: DeprecationWarning: 'audioop' is deprecated and slated for removal in Python 3.13
  import audioop
Traceback (most recent call last):
  File "/home/rodrigo/Proyectos/marta/Call-Services/test/performance_test/testNvidiaRivaTTS.py", line 169, in <module>
    for transformed_audio in ulaw_response_audio_stream:
  File "/home/rodrigo/Proyectos/marta/Call-Services/test/performance_test/testNvidiaRivaTTS.py", line 152, in wav_converter
    for raw_wav in raw_stream:
  File "/home/rodrigo/Proyectos/marta/Call-Services/test/performance_test/testNvidiaRivaTTS.py", line 130, in generate_audio
    resp = tts_service.synthesize(**req)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/rodrigo/Proyectos/marta/Call-Services/venv/lib/python3.11/site-packages/riva/client/tts.py", line 91, in synthesize
    add_custom_dictionary_to_config(req, custom_dictionary)
  File "/home/rodrigo/Proyectos/marta/Call-Services/venv/lib/python3.11/site-packages/riva/client/tts.py", line 15, in add_custom_dictionary_to_config
    result_list = [f"{key}  {value}" for key, value in custom_dictionary.items()]
                                                       ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'items'

As a temporary workaround, the issue can be avoided by passing an empty dictionary as custom_dictionary={} in the request.

Please take more care with releases to avoid breaking changes like this in the future.

Thank you!

Start with riva/client/tts.py, especially synthesize() and add_custom_dictionary_to_config(), using the traceback and the request that omits custom_dictionary. Reproduce the failure with tts.synthesize() and verify that the call succeeds without the workaround of passing custom_dictionary={}.