fix: fix -1 offset lookups failing

Author: TrevorBergeron

bigframes/core/indexers.py

@@ -402,7 +402,8 @@ def _iloc_getitem_series_or_dataframe(
     pd.Series,
 ]:
     if isinstance(key, int):
-        internal_slice_result = series_or_dataframe._slice(key, key + 1, 1)
+        stop_key = key + 1 if key != -1 else None
+        internal_slice_result = series_or_dataframe._slice(key, stop_key, 1)
         result_pd_df = internal_slice_result.to_pandas()
         if result_pd_df.empty:
             raise IndexError("single positional indexer is out-of-bounds")

tests/system/small/test_series.py

@@ -216,7 +216,7 @@ def test_series___getitem__(scalars_dfs, index_col, key):
 def test_series___getitem___with_int_key(scalars_dfs):
     col_name = "int64_too"
     index_col = "string_col"
-    key = 2
+    key = -1
     scalars_df, scalars_pandas_df = scalars_dfs
     scalars_df = scalars_df.set_index(index_col, drop=False)
     scalars_pandas_df = scalars_pandas_df.set_index(index_col, drop=False)