Don't consider results as modified if they're empty.

core/results.py

@@ -114,7 +114,7 @@ class Results(Markable):
                 self.__group_of_duplicate[dupe] = g
                 if not hasattr(dupe, 'is_ref'):
                     dupe.is_ref = False
-        self.is_modified = True
+        self.is_modified = bool(self.__groups)
         old_filters = nonone(self.__filters, [])
         self.apply_filter(None)
         for filter_str in old_filters:

core/tests/results_test.py

@@ -72,6 +72,11 @@ class TestCaseResultsEmpty:
     def test_is_modified(self):
         assert not self.results.is_modified
     
+    def test_is_modified_after_setting_empty_group(self):
+        # Don't consider results as modified if they're empty
+        self.results.groups = []
+        assert not self.results.is_modified
+    
 
 class TestCaseResultsWithSomeGroups:
     def setup_method(self, method):