fix(api): avoid memory leak with click functions in nvim_eval_statusline() (#21845)

Problem:    Allocated click function memory is lost due to
            `nvim_eval_statusline()` not passing in a `StlClickRecord`.
Solution:   Do not allocate click function memory if `tabtab == NULL`.

Resolve #21764, supersede #21842.

test/functional/api/vim_spec.lua

@@ -3211,6 +3211,17 @@ describe('API', function()
             'TextWithNoHighlight%#WarningMsg#TextWithWarningHighlight',
             { use_winbar = true, highlights = true }))
       end)
+      it('no memory leak with click functions', function()
+        meths.eval_statusline('%@ClickFunc@StatusLineStringWithClickFunc%T', {})
+        eq({
+            str = 'StatusLineStringWithClickFunc',
+            width = 29
+          },
+          meths.eval_statusline(
+            '%@ClickFunc@StatusLineStringWithClickFunc%T',
+            {})
+          )
+      end)
     end)
   end)
   describe('nvim_parse_cmd', function()