Incorrect SUM() result on constant IF expression with BLOB column (returns 48 instead of 0)

## Bug Report

### 1. Minimal reproduce step (Required)

drop table if exists t1;
create table t1
(
c0 tinyblob not null
);

INSERT INTO t1 (c0) VALUES (0x31343936393730313331);

SELECT SUM((IF(t1.c0 IS NULL, t1.c0, 0)))
FROM t1;

### 2. What did you expect to see? (Required)

0

Reason:

The column c0 is defined as NOT NULL, so the condition:
t1.c0 IS NULL

is always FALSE.

Therefore, the expression:
IF(t1.c0 IS NULL, t1.c0, 0)
always evaluates to:
0
So the query is equivalent to:
SELECT SUM(0) FROM t1;
which should return:
0

### 3. What did you see instead (Required)

48

### 4. What is your TiDB version? (Required)

Release Version: v8.5.5
Edition: Community
Git Commit Hash: 1fa258b833ff113883beeba40bc130be7ce66610
Git Branch: HEAD
UTC Build Time: 2026-01-14 22:19:19
GoVersion: go1.25.5
Race Enabled: false
Check Table Before Drop: false
Store: tikv