Add test to check _collapse skips null values

Signed-off-by: John Pennycook <[email protected]>

tests/data/test_projection.py

@@ -1,10 +1,12 @@
 # Copyright (C) 2022-2023 Intel Corporation
 # SPDX-License-Identifier: MIT
 
+import unittest
+
 import pandas as pd
+
 from p3.data import projection
 from p3.data._projection import _collapse
-import unittest
 
 
 class TestProjection(unittest.TestCase):
@@ -24,6 +26,18 @@ def test_collapse(self):
 
         pd.testing.assert_frame_equal(df, expected_df)
 
+    def test_collapse_null(self):
+        """Check that _collapse skips null values"""
+        data = {"c1": ["x", "y", "z"], "c2": ["1", None, "3"]}
+        df = pd.DataFrame(data)
+
+        _collapse(df, ["c1", "c2"], "c3")
+
+        expected_data = {"c3": ["x-1", "y", "z-3"]}
+        expected_df = pd.DataFrame(expected_data)
+
+        pd.testing.assert_frame_equal(df, expected_df)
+
     def test_required_columns(self):
         """p3.data.projection.required_columns"""
         df = pd.DataFrame()