fix(api): parse_expression crash with unopened ] and node

Problem: nvim_parse_expression null pointer dereference with unmatched ]
followed by a node.

Solution: if ast_stack was empty, set new_top_node_p to top of the stack after
pushing the list literal node; similar to what's done for curlies.

This bug was originally found by a Matrix user, but I couldn't remember how to
trigger it... Ran into the other crash while finding a repro. :P

test/functional/api/vim_spec.lua

@@ -3242,6 +3242,8 @@ describe('API', function()
       api.nvim_input(':<C-r>=')
       api.nvim_input('1bork/') -- #29648
       assert_alive()
+      api.nvim_input('<C-u>];')
+      assert_alive()
       api.nvim_parse_expression('a{b}', '', false)
       assert_alive()
     end)