patch 8.2.3249: Vim9: error for re-imported function with default argument

Problem:    Vim9: error for re-imported function with default argument.
Solution:   Do not check argument type if it is still unknown. (closes #8653)

src/testdir/test_vim9_script.vim

@@ -1626,6 +1626,9 @@ def Test_vim9script_reload_noclear()
   var lines =<< trim END
     vim9script
     export var exported = 'thexport'
+
+    export def TheFunc(x = 0)
+    enddef
   END
   writefile(lines, 'XExportReload')
   lines =<< trim END
@@ -1638,6 +1641,9 @@ def Test_vim9script_reload_noclear()
       return 'again'
     enddef
 
+    import TheFunc from './XExportReload'
+    TheFunc()
+
     if exists('s:loaded') | finish | endif
     var s:loaded = true