jinja : handle empty expressions correctly (#20913)

* Reject empty computed member expressions before returning slices[0] from parse_member_expression_arguments().

* Treat empty computed member expressions with Jinja2 undefined semantics

Treat empty computed member expressions like `a[]` as undefined instead of
raising a parser error, to match Jinja2 behavior.

- return a noop expression for empty computed member arguments
- return undefined when a computed member key evaluates to undefined
- add Jinja tests covering `a[]|default('fallback')` and `a[] is undefined`

* Handle undefined computed member properties

Move undefined-property handling to the common member access path, and add a test covering `a[undefined] is undefined`.

* Use default undefined value in member access

Initialize val and then return it when property is undefined.

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

* empty statement parses to blank_expression instead of noop_statement

---------

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

tests/test-jinja.cpp

@@ -387,6 +387,24 @@ static void test_expressions(testing & t) {
         "Bob"
     );
 
+    test_template(t, "empty computed member defaults to undefined",
+        "{{ a[]|default('fallback') }}",
+        {{"a", {{"name", "Bob"}}}},
+        "fallback"
+    );
+
+    test_template(t, "empty computed member is undefined",
+        "{{ a[] is undefined }}",
+        {{"a", {{"name", "Bob"}}}},
+        "True"
+    );
+
+    test_template(t, "undefined computed member is undefined",
+        "{{ a[undefined] is undefined }}",
+        {{"a", {{"name", "Bob"}}}},
+        "True"
+    );
+
     test_template(t, "array access",
         "{{ items[1] }}",
         {{"items", json::array({"a", "b", "c"})}},