Add ValueError tests for nighit (PR 1377)

PR  #1377 adds guardrails to nighit().
This PR add tests to those guardrails.

Signed-off-by: Arthit Suriyawongkul <arthit@gmail.com>

Author: bact

tests/core/test_morpheme.py

@@ -10,15 +10,26 @@
 class MorphemeTestCase(unittest.TestCase):
     def test_nighit(self):
         self.assertEqual(nighit("สํ", "คีต"), "สังคีต")
+        self.assertEqual(
+            nighit("สํ", "คีต "), "สังคีต"
+        )  # w2 has trailing space, should still work
+        self.assertEqual(
+            nighit("สํ ", "คีต"), "สังคีต"
+        )  # w1 has trailing space, should still work
         self.assertEqual(nighit("สํ", "จร"), "สัญจร")
         self.assertEqual(nighit("สํ", "ฐาน"), "สัณฐาน")
         self.assertEqual(nighit("สํ", "นิษฐาน"), "สันนิษฐาน")
         self.assertEqual(nighit("สํ", "ปทา"), "สัมปทา")
         self.assertEqual(nighit("สํ", "โยค"), "สังโยค")
+        self.assertEqual(nighit("", "คีต"), "คีต")  # w1 is empty, should return w2
+        self.assertEqual(nighit("สํ", ""), "สํ")  # w2 is empty, should return w1
+
         with self.assertRaises(NotImplementedError):
             nighit("abc", "คีต")  # w1 does not end with ํ and len > 2
         with self.assertRaises(NotImplementedError):
             nighit("สํ", "มาร")  # consonant ม is not in any supported group
+        with self.assertRaises(ValueError):
+            nighit("สํ", "123")  # w2 does not contain any Thai consonant
 
     def test_is_native_thai(self):
         self.assertFalse(is_native_thai(None))  # type: ignore[arg-type]