[3.12] gh-116325: Raise `SyntaxError` rather than `IndexError` on ForwardRef with empty string arg (GH-116341) (#116347)

gh-116325: Raise `SyntaxError` rather than `IndexError` on ForwardRef with empty string arg (GH-116341)
(cherry picked from commit a29998a06bf75264c3faaeeec4584a5f75b45a1f)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>