fix(api): parse_expression crash with ident and curly

Problem: nvim_parse_expression null pointer dereference when parsing an
identifier followed by { with "highlight" parameter set to false.

Solution: only set opening_hl_idx if pstate->colors is not NULL.

Not added to parser_tests.lua as that uses highlight = true.

test/functional/api/vim_spec.lua

@@ -3238,9 +3238,11 @@ describe('API', function()
       end
     end
 
-    it('does not crash parsing invalid VimL expression #29648', function()
+    it('does not crash parsing invalid VimL expression', function()
       api.nvim_input(':<C-r>=')
-      api.nvim_input('1bork/')
+      api.nvim_input('1bork/') -- #29648
+      assert_alive()
+      api.nvim_parse_expression('a{b}', '', false)
       assert_alive()
     end)