Set prompt2 & prompt3 default to None as indicated by parameter args

[DavidCarlyn]

What does this PR do?

Sets the default parameters of StableDiffusion3Pipeline.encode_prompt for prompt2 & prompt3 to None. Otherwise, if the encode_prompt is directly used, then an error for require arguments will be thrown.

The parameter documentation indicates that they should handled being None since they will default to prompt:

prompt (`str` or `List[str]`, *optional*):
      prompt to be encoded
prompt_2 (`str` or `List[str]`, *optional*):
       The prompt or prompts to be sent to the `tokenizer_2` and `text_encoder_2`. If not defined, `prompt` is
       used in all text-encoders
prompt_3 (`str` or `List[str]`, *optional*):
       The prompt or prompts to be sent to the `tokenizer_3` and `text_encoder_3`. If not defined, `prompt` is
       used in all text-encoders

Also confirmed by L429-L434:

if prompt_embeds is None:
    prompt_2 = prompt_2 or prompt
    prompt_2 = [prompt_2] if isinstance(prompt_2, str) else prompt_2

    prompt_3 = prompt_3 or prompt
    prompt_3 = [prompt_3] if isinstance(prompt_3, str) else prompt_3