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>

common/jinja/runtime.cpp

@@ -771,10 +771,15 @@ value member_expression::execute_impl(context & ctx) {
     }
 
     JJ_DEBUG("Member expression on object type %s, property type %s", object->type().c_str(), property->type().c_str());
-    ensure_key_type_allowed(property);
-
     value val = mk_val<value_undefined>("object_property");
 
+    if (property->is_undefined()) {
+        JJ_DEBUG("%s", "Member expression property is undefined, returning undefined");
+        return val;
+    }
+
+    ensure_key_type_allowed(property);
+
     if (is_val<value_undefined>(object)) {
         JJ_DEBUG("%s", "Accessing property on undefined object, returning undefined");
         return val;