fix: correctly handle null values when initializing fingerprint ordering (#210)

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Fixes #<issue_number_goes_here> 🦕

Author: TrevorBergeron

bigframes/session/__init__.py

@@ -1120,8 +1120,9 @@ def _create_total_ordering(
         ordering_hash_part = guid.generate_guid("bigframes_ordering_")
         ordering_rand_part = guid.generate_guid("bigframes_ordering_")
 
+        # All inputs into hash must be non-null or resulting hash will be null
         str_values = list(
-            map(lambda col: _convert_to_string(table[col]), table.columns)
+            map(lambda col: _convert_to_nonnull_string(table[col]), table.columns)
         )
         full_row_str = (
             str_values[0].concat(*str_values[1:])
@@ -1419,7 +1420,7 @@ def _can_cluster_bq(field: bigquery.SchemaField):
     )
 
 
-def _convert_to_string(column: ibis_types.Column) -> ibis_types.StringColumn:
+def _convert_to_nonnull_string(column: ibis_types.Column) -> ibis_types.StringValue:
     col_type = column.type()
     if (
         col_type.is_numeric()
@@ -1436,4 +1437,6 @@ def _convert_to_string(column: ibis_types.Column) -> ibis_types.StringColumn:
         # TO_JSON_STRING works with all data types, but isn't the most efficient
         # Needed for JSON, STRUCT and ARRAY datatypes
         result = vendored_ibis_ops.ToJsonString(column).to_expr()  # type: ignore
-    return typing.cast(ibis_types.StringColumn, result)
+    # Escape backslashes and use backslash as delineator
+    escaped = typing.cast(ibis_types.StringColumn, result.fillna("")).replace("\\", "\\\\")  # type: ignore
+    return typing.cast(ibis_types.StringColumn, ibis.literal("\\")).concat(escaped)

tests/system/small/test_dataframe.py

@@ -2703,6 +2703,14 @@ def test_sample(scalars_dfs, frac, n, random_state):
     assert bf_result.shape[1] == scalars_df.shape[1]
 
 
+def test_sample_determinism(penguins_df_default_index):
+    df = penguins_df_default_index.sample(n=100, random_state=12345).head(15)
+    bf_result = df.to_pandas()
+    bf_result2 = df.to_pandas()
+
+    pandas.testing.assert_frame_equal(bf_result, bf_result2)
+
+
 def test_sample_raises_value_error(scalars_dfs):
     scalars_df, _ = scalars_dfs
     with pytest.raises(