FIX: `MultiModalMessage` in gemini with openai sdk error occured

[SongChiYoung]

Why are these changes needed?

Multimodal message fill context with other routine. However current _set_empty_to_whitespace is fill with context. So, error occured.

And, I checked multimodal_user_transformer_funcs and I found it, in this routine, context must not be empty. Now remove the _set_empty_to_whitespace when multimodal message,

Related issue number

#6439

Checks

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• [x] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [x] I've made sure all auto checks have passed.