ag-ui-adk validation error for TextMessageContentEvent if combined_text is empty

[BnnaFish]

Delta could be empty string. And there is a check for this scenario

But previously TextMessageContentEvent requires non-empty string and there is no filter for that. So pydentic error is raised

Empty text is used for stream event, e.g. finish_reason=<FinishReason.STOP: 'STOP'>
So this is a valid event.

[contextablemark]

Should be addressed by #745