jinja : handle empty expressions correctly (#20913)

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 <redacted>
* empty statement parses to blank_expression instead of noop_statement

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Co-authored-by: Sigbjørn Skjæret <redacted>