patch 8.2.1300: Vim9: optional argument type not parsed properly

Problem:    Vim9: optional argument type not parsed properly.
Solution:   Skip over the "?". (issue #6507)
diff --git a/src/userfunc.c b/src/userfunc.c
index 3e566f3..d17e807 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -123,7 +123,7 @@
 		return arg;
 	    }
 	    type = skipwhite(p);
-	    p = skip_type(type);
+	    p = skip_type(type, TRUE);
 	    type = vim_strnsave(type, p - type);
 	}
 	else if (*skipwhite(p) != '=')
@@ -2778,7 +2778,7 @@
 	if (*p == ':')
 	{
 	    ret_type = skipwhite(p + 1);
-	    p = skip_type(ret_type);
+	    p = skip_type(ret_type, FALSE);
 	    if (p > ret_type)
 	    {
 		ret_type = vim_strnsave(ret_type, p - ret_type);