TST: Add test for missing values in relabel.

tests/pipeline/test_classifier.py

@@ -513,6 +513,58 @@ class ClassifierTestCase(BasePipelineTestCase):
             mask=self.build_mask(self.ones_mask(shape=data.shape)),
         )
 
+    @parameter_space(
+        __fail_fast=True,
+        missing_value=[None, 'M'],
+    )
+    def test_relabel_missing_value_interactions(self, missing_value):
+
+        mv = missing_value
+
+        class C(Classifier):
+            inputs = ()
+            dtype = categorical_dtype
+            missing_value = mv
+            window_length = 0
+
+        c = C()
+
+        def relabel_func(s):
+            if s == 'B':
+                return mv
+            return ''.join([s, s])
+
+        raw = np.asarray(
+            [['A', 'B', 'C', mv],
+             [mv, 'A', 'B', 'C'],
+             ['C', mv, 'A', 'B'],
+             ['B', 'C', mv, 'A']],
+            dtype=categorical_dtype,
+        )
+        data = LabelArray(raw, missing_value=mv)
+
+        expected_relabeled_raw = np.asarray(
+            [['AA', mv, 'CC', mv],
+             [mv, 'AA', mv, 'CC'],
+             ['CC', mv, 'AA', mv],
+             [mv, 'CC', mv, 'AA']],
+            dtype=categorical_dtype,
+        )
+
+        terms = {
+            'relabeled': c.relabel(relabel_func),
+        }
+        expected_results = {
+            'relabeled': LabelArray(expected_relabeled_raw, missing_value=mv),
+        }
+
+        self.check_terms(
+            terms,
+            expected_results,
+            initial_workspace={c: data},
+            mask=self.build_mask(self.ones_mask(shape=data.shape)),
+        )
+
     def test_relabel_int_classifier_not_yet_supported(self):
         class C(Classifier):
             inputs = ()