MDEV-29242: Assertion `computed_weight == weight' failed SEL_ARG::verify_weight

Make SEL_ARG::make_root() maintain SEL_ARG::weight.

Also, an unrelated change: fix dbug_print_sel_arg() to correctly
print SQL NULL for the right endpoint.

mysql-test/main/range_notembedded.result

@@ -248,3 +248,12 @@ WHERE
 (help_topic_id < '2' OR help_topic_id != 8 OR help_topic_id < 1) AND
 help_keyword_id = help_topic_id;
 help_topic_id	help_keyword_id
+#
+# MDEV-29242: Assertion `computed_weight == weight' failed SEL_ARG::verify_weight
+#
+CREATE TABLE t1 (id INT, KEY (id));
+INSERT INTO t1 VALUES (1),(5);
+SELECT id FROM t1 WHERE id IS NULL OR id NOT BETWEEN 1 AND 4;
+id
+5
+DROP TABLE t1;

mysql-test/main/range_notembedded.test

@@ -150,3 +150,12 @@ FROM mysql.help_relation
 WHERE
   (help_topic_id < '2' OR help_topic_id != 8 OR help_topic_id < 1) AND
   help_keyword_id = help_topic_id;
+
+--echo #
+--echo # MDEV-29242: Assertion `computed_weight == weight' failed SEL_ARG::verify_weight
+--echo #
+CREATE TABLE t1 (id INT, KEY (id));
+INSERT INTO t1 VALUES (1),(5);
+SELECT id FROM t1 WHERE id IS NULL OR id NOT BETWEEN 1 AND 4;
+DROP TABLE t1;
+

sql/opt_range.cc

@@ -1901,7 +1901,9 @@ inline void SEL_ARG::make_root()
   left=right= &null_element;
   color=BLACK;
   next=prev=0;
-  use_count=0; elements=1;
+  use_count=0;
+  elements=1;
+  weight= 1 + (next_key_part? next_key_part->weight : 0);
 }
 
 SEL_ARG::SEL_ARG(Field *f,const uchar *min_value_arg,
@@ -16153,6 +16155,7 @@ const char *dbug_print_sel_arg(SEL_ARG *sel_arg)
     out.append("+inf");
   else
   {
+    buf.length(0);
     print_sel_arg_key(sel_arg->field, sel_arg->max_value, &buf);
     out.append(buf);
   }