patch 8.2.1848: crash when passing a NULL string or list to popup_settext()

Problem:    Crashyyyyy passing a NULL string or list to popup_settext().
Solution:   Check for NULL pointers. (closes #7132)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 187b4f1..1524e78 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -2363,6 +2363,16 @@
   call popup_close(id)
 endfunc
 
+func Test_popup_settext_null()
+  let id = popup_create('', #{ tabpage: 0 })
+  call popup_settext(id, test_null_list())
+  call popup_close(id)
+
+  let id = popup_create('', #{ tabpage: 0 })
+  call popup_settext(id, test_null_string())
+  call popup_close(id)
+endfunc
+
 func Test_popup_hidden()
   new