fix: fix error in Series.drop(0) (#575)

Due to implicit 0 non-truthfulness, 0 was getting erroneously converted to None.

bigframes/series.py

@@ -351,9 +351,11 @@ def drop(
         columns: Union[blocks.Label, typing.Iterable[blocks.Label]] = None,
         level: typing.Optional[LevelType] = None,
     ) -> Series:
-        if labels and index:
-            raise ValueError("Must specify exacly one of 'labels' or 'index'")
-        index = labels or index
+        if (labels is None) == (index is None):
+            raise ValueError("Must specify exactly one of 'labels' or 'index'")
+
+        if labels is not None:
+            index = labels
 
         # ignore axis, columns params
         block = self._block

tests/system/small/test_series.py

@@ -1529,10 +1529,16 @@ def test_groupby_window_ops(scalars_df_index, scalars_pandas_df_index, operator)
     )
 
 
-def test_drop_label(scalars_df_index, scalars_pandas_df_index):
-    col_name = "int64_col"
-    bf_series = scalars_df_index[col_name].drop(1).to_pandas()
-    pd_series = scalars_pandas_df_index[col_name].drop(1)
+@pytest.mark.parametrize(
+    ("label", "col_name"),
+    [
+        (0, "bool_col"),
+        (1, "int64_col"),
+    ],
+)
+def test_drop_label(scalars_df_index, scalars_pandas_df_index, label, col_name):
+    bf_series = scalars_df_index[col_name].drop(label).to_pandas()
+    pd_series = scalars_pandas_df_index[col_name].drop(label)
     pd.testing.assert_series_equal(
         pd_series,
         bf_series,