change speaker id to be str to include nones

voxpopuli.py

@@ -101,7 +101,7 @@ class Voxpopuli(datasets.GeneratorBasedBuilder):
                 "raw_text": datasets.Value("string"),
                 "normalized_text": datasets.Value("string"),
                 "gender": datasets.Value("string"),  # TODO: ClassVar?
-                "speaker_id": datasets.Value("int64"),
+                "speaker_id": datasets.Value("string"),
                 "is_gold_transcript": datasets.Value("bool"),
                 "accent": datasets.Value("string"),
                 "audio": datasets.Audio(sampling_rate=16_000),