patch 8.2.3751: cannot assign a lambda to an option that takes a function

Problem:    Cannot assign a lambda to an option that takes a function.
Solution:   Automatically convert the lambda to a string. (Yegappan
            Lakshmanan, closes #9286)
diff --git a/src/spell.c b/src/spell.c
index f472f99..9b2d3e8 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -3830,7 +3830,7 @@
 
     if (no_spell_checking(curwin))
 	return;
-    (void)get_option_value((char_u*)"spl", &dummy, &spl, OPT_LOCAL);
+    (void)get_option_value((char_u*)"spl", &dummy, &spl, NULL, OPT_LOCAL);
 
     // Create a new empty buffer in a new window.
     do_cmdline_cmd((char_u *)"new");