fix AttributeError in NVIDIA Riva STT

[gau-nernst]

I'm getting AttributeError here

agents/livekit-plugins/livekit-plugins-nvidia/livekit/plugins/nvidia/stt.py

Lines 289 to 291 in 8086121

	text=word.get("word", ""),
	start_time=word.get("start_time", 0) + self.start_time_offset,
	end_time=word.get("end_time", 0) + self.start_time_offset,

The safer way is to use getattr(), which I see that it is already used in a few lines earlier

agents/livekit-plugins/livekit-plugins-nvidia/livekit/plugins/nvidia/stt.py

Lines 278 to 279 in 8086121

	start_time = getattr(words[0], "start_time", 0) / 1000.0 + self.start_time_offset
	end_time = getattr(words[-1], "end_time", 0) / 1000.0 + self.start_time_offset

Pinging @chenghao-mou for review since it looks like you made the last commit to the file. Thank you!

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