fix TNL-767 When students click "check" without submitting an answer, our basic problem types respond in inconsistent ways.

common/lib/capa/capa/responsetypes.py

@@ -2134,6 +2134,10 @@ class StringResponse(LoncapaResponse):
         Note: for old code, which supports _or_ separator, we add some  backward compatibility handling.
         Should be removed soon. When to remove it, is up to Lyla Fisher.
         """
+        # if given answer is empty.
+        if not given:
+            return False
+
         _ = self.capa_system.i18n.ugettext
         # backward compatibility, should be removed in future.
         if self.backward:

common/lib/capa/capa/tests/test_responsetypes.py

@@ -946,6 +946,13 @@ class StringResponseTest(ResponseTest):  # pylint: disable=missing-docstring
         hint = correct_map.get_hint('1_2_1')
         self.assertEqual(hint, self._get_random_number_result(problem.seed))
 
+    def test_empty_answer_graded_as_incorrect(self):
+        """
+        Tests that problem should be graded incorrect if blank space is chosen as answer
+        """
+        problem = self.build_problem(answer=" ", case_sensitive=False, regexp=True)
+        self.assert_grade(problem, u" ", "incorrect")
+
 
 class CodeResponseTest(ResponseTest):  # pylint: disable=missing-docstring
     xml_factory_class = CodeResponseXMLFactory