ref: fix types for sentry.api.serializers.snuba (#79752)

<!-- Describe your PR here. -->

pyproject.toml

@@ -189,7 +189,6 @@ module = [
     "sentry.api.serializers.rest_framework.mentions",
     "sentry.api.serializers.rest_framework.notification_action",
     "sentry.api.serializers.rest_framework.rule",
-    "sentry.api.serializers.snuba",
     "sentry.auth.helper",
     "sentry.auth.provider",
     "sentry.auth.system",

src/sentry/api/serializers/snuba.py

@@ -81,7 +81,7 @@ class SnubaTSResultSerializer(BaseSnubaSerializer):
             (key, list(group))
             for key, group in itertools.groupby(result.data["data"], key=lambda r: r["time"])
         ]
-        attrs = None
+        attrs = {}
         if self.lookup:
             attrs = self.get_attrs(
                 [value_from_row(r, self.lookup.columns) for _, v in data for r in v]