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

[windtalker]

Bug Report

Please answer these questions before submitting your issue. Thanks!

In

tiflash/dbms/src/Functions/FunctionsString.cpp

Lines 3588 to 3591 in 78bd3f0

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

And

tiflash/dbms/src/Functions/FunctionsString.cpp

Lines 3662 to 3665 in 78bd3f0

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

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)

[xzhangxian1008]

/assign