Processing Utils: continue when content is a string

[RyanMullins]

What does this PR do?

Fixes crash for the following messages structure when calling

processor = AutoProcessor.from_pretrained(model_id)
messages = [
    {"role": "system", "content": "You are a helpful assistant."},
    {"role": "user", "content": [{"type": "text", "text": "Write a short joke about saving RAM."}]},
]
processor.apply_chat_template(messages, tokenize=True)

Traceback (most recent call last):
  File "$HOME/git/transformers/local_testing/assisted_generation.py", line 18, in <module>
    inputs = processor.apply_chat_template(
        messages,
    ...<4 lines>...
        enable_thinking=False,
    )
  File "$HOME/git/transformers/src/transformers/processing_utils.py", line 1822, in apply_chat_template
    content_block for content_block in content if content_block["type"] in ["image", "video"]
                                                  ~~~~~~~~~~~~~^^^^^^^^
TypeError: string indices must be integers, not 'str'

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