Return empty string when content is blocked for Gemini

[JosselinSomervilleRoberts]

No description provided.

[yifanmai]

Actually can we return something like {"predictions": null} instead, so that we can distinguish between actual and "placeholder" empty predictions?

Then later on you can do

if response["predictions"] is None:
    return RequestResult(
        success=False,
        cached=False,
        error="Response was empty due to content moderation filter",
        completions=[],
        embedding=[],
        error_flags=ErrorFlags(is_retriable=False, is_fatal=False),
        request_time=response["request_time"],
        request_datetime=response["request_datetime"],
    )

which matches what we do for Palmyra and will produce the "WARNING: Non-fatal error treated as empty completion" warning log message here.