SIENTIAPDE-1579?: removed duplicated method

tests/utils/repository/test_training_repository.py

@@ -1127,9 +1127,10 @@ class TestExtractModelEquationPolynomial:
 class TestFrontendFormatToStrftime:
     """Tests for _frontend_format_to_strftime."""
 
-    def test_empty_returns_unchanged(self):
-        """Empty string is returned as-is."""
-        assert _frontend_format_to_strftime('') == ''
+    def test_empty_or_none_returns_none(self):
+        """Empty string or None returns None (same behavior as models module)."""
+        assert _frontend_format_to_strftime('') is None
+        assert _frontend_format_to_strftime(None) is None
 
     def test_dd_mm_yyyy_hh_mm_ss(self):
         """Converts dd/MM/yyyy HH:mm:ss to strftime."""