fix crash on empty sexp in test, ()
fix crash on empty sexp in test, ()

Now this no longer crashes:

=====
a test
=====
-----
()

...

tree-sitter test:

1 failure:

expected / actual

  1. a test:

    (source_file)

    ()

fixes #1537

tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter