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.h

@@ -263,6 +263,14 @@ struct comment_statement : public statement {
 
 // Expressions
 
+// Represents an omitted expression in a computed member, e.g. `a[]`.
+struct blank_expression : public expression {
+    std::string type() const override { return "BlankExpression"; }
+    value execute_impl(context &) override {
+        return mk_val<value_undefined>();
+    }
+};
+
 struct member_expression : public expression {
     statement_ptr object;
     statement_ptr property;