`lpad/rpad` return wrong result if it need return null for not null input

## Bug Report

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In https://github.com/pingcap/tiflash/blob/78bd3f04dc5f10b94aef13cf3d9e0d42a4c808e5/dbms/src/Functions/FunctionsString.cpp#L3588-L3591
And 
https://github.com/pingcap/tiflash/blob/78bd3f04dc5f10b94aef13cf3d9e0d42a4c808e5/dbms/src/Functions/FunctionsString.cpp#L3662-L3665

It returns `true`(which means the result is `null`) without adding the tailing `\0`
### 1. Minimal reproduce step (Required)



### 2. What did you expect to see? (Required)

### 3. What did you see instead (Required)

### 4. What is your TiFlash version? (Required)

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`lpad/rpad` return wrong result if it need return null for not null input #9465

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	if (target_len < 0 \|\| (data_len < static_cast<ColumnString::Offset>(target_len) && pad_len == 0))
	{
	return true;
	}

lpad/rpad return wrong result if it need return null for not null input #9465

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Bug Report

1. Minimal reproduce step (Required)

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

3. What did you see instead (Required)

4. What is your TiFlash version? (Required)

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`lpad/rpad` return wrong result if it need return null for not null input #9465