jinja: treat a null left operand of in as a plain lookup (#28620)

Templates that default an optional variable to none and then test its
membership in a map hit an error, while the same expression is a normal
lookup returning false in Jinja. The undefined counterpart of this case
was already handled just above.

tests/test-jinja.cpp

@@ -374,6 +374,24 @@ static void test_expressions(testing & t) {
         "42"
     );
 
+    test_template(t, "none in object",
+        "{{ x in {'low': 1, 'high': 2} }}",
+        {{"x", nullptr}},
+        "False"
+    );
+
+    test_template(t, "none not in object",
+        "{{ x not in {'low': 1, 'high': 2} }}",
+        {{"x", nullptr}},
+        "True"
+    );
+
+    test_template(t, "none in array",
+        "{{ x in [1, none, 3] }}",
+        {{"x", nullptr}},
+        "True"
+    );
+
     test_template(t, "dot notation",
         "{{ user.name }}",
         {{"user", {{"name", "Bob"}}}},