fix null label handling

Author: TrevorBergeron

bigframes/core/utils.py

@@ -119,7 +119,7 @@ def get_standardized_ids(
     ]
     idx_ids = [
         UNNAMED_INDEX_ID
-        if idx_label is None
+        if pd.isna(idx_label)  # type: ignore
         else label_to_identifier(idx_label, strict=strict)
         for idx_label in idx_labels
     ]

tests/unit/core/compile/sqlglot/snapshots/test_compile_fromrange/test_compile_fromrange/out.sql

@@ -157,7 +157,7 @@ WITH `bfcte_6` AS (
 SELECT
   CAST(TIMESTAMP_MICROS(
     CAST(CAST(`bfcol_41` AS BIGNUMERIC) * 7000000 + CAST(UNIX_MICROS(CAST(CAST(`bfcol_44` AS DATE) AS TIMESTAMP)) AS BIGNUMERIC) AS INT64)
-  ) AS DATETIME) AS `nan`,
+  ) AS DATETIME) AS `bigframes_unnamed_index`,
   `bfcol_55` AS `int64_col`,
   `bfcol_56` AS `int64_too`
 FROM `bfcte_30`