MDEV-23264 Unique blobs allow duplicate values upon UPDATE

Problem:-
  We are able to insert duplicate value in table because cmp_binary_offset
  is not able to differentiate between NULL and empty string. So
  check_duplicate_long_entry_key is never called and we don't check for
  duplicate.
Solution
  Added a if condition with is_null() on field which can differentiate
  between NULL and empty string.

mysql-test/main/long_unique_bugs.result

@@ -314,6 +314,15 @@ ERROR 23000: Duplicate entry '1' for key 'v2'
 update t1,t2 set v1 = v2 , v5 = 0;
 ERROR 23000: Duplicate entry '-128' for key 'v1'
 drop table t1, t2;
+CREATE TABLE t1 (f TEXT UNIQUE);
+INSERT INTO t1 VALUES (NULL),(NULL);
+UPDATE t1 SET f = '';
+ERROR 23000: Duplicate entry '' for key 'f'
+SELECT * FROM t1;
+f
+
+NULL
+DROP TABLE t1;
 #
 # MDEV-21540 Initialization of already inited long unique index on reorganize partition
 #

mysql-test/main/long_unique_bugs.test

@@ -396,6 +396,17 @@ update t1 set v2 = 1, v3 = -128;
 update t1,t2 set v1 = v2 , v5 = 0;
 drop table t1, t2;
 
+#
+# MDEV-23264 Unique blobs allow duplicate values upon UPDATE
+#
+
+CREATE TABLE t1 (f TEXT UNIQUE);
+INSERT INTO t1 VALUES (NULL),(NULL);
+--error ER_DUP_ENTRY
+UPDATE t1 SET f = '';
+SELECT * FROM t1;
+DROP TABLE t1;
+
 --echo #
 --echo # MDEV-21540 Initialization of already inited long unique index on reorganize partition
 --echo #

sql/handler.cc

@@ -6750,8 +6750,13 @@ static int check_duplicate_long_entries_update(TABLE *table, handler *h, uchar *
       for (uint j= 0; j < key_parts; j++, keypart++)
       {
         field= keypart->field;
-        /* Compare fields if they are different then check for duplicates*/
-        if(field->cmp_binary_offset(reclength))
+        /*
+          Compare fields if they are different then check for duplicates
+          cmp_binary_offset cannot differentiate between null and empty string
+          So also check for that too
+        */
+        if((field->is_null(0) != field->is_null(reclength)) ||
+                               field->cmp_binary_offset(reclength))
         {
           if((error= check_duplicate_long_entry_key(table, table->update_handler,
                                                  new_rec, i)))