fix(expr): fix lpad and rpad on empty fill string

[wangrunji0408]

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What's changed and what's your intention?

fix #8949

Checklist For Contributors

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• I have demonstrated that backward compatibility is not broken by breaking changes and created issues to track deprecated features to be removed in the future. (Please refer to the issue)
• All checks passed in ./risedev check (or alias, ./risedev c)

Checklist For Reviewers

• I have requested macro/micro-benchmarks as this PR can affect performance substantially, and the results are shown.

Documentation

Click here for Documentation

Types of user-facing changes

Please keep the types that apply to your changes, and remove the others.

• SQL commands, functions, and operators

Release note

Note that for lpad and rpad function:

lpad(string text, length int, fill text)
rpad(string text, length int, fill text)

If fill is given an empty string and length > length of string, the function will return string directly.

[xiangjinwu]

We can uncomment the regress tests as well:

risingwave/src/tests/regress/data/sql/strings.sql

Lines 715 to 721 in 30dca71

	--@ SELECT lpad('hi', 5, '');
	SELECT rpad('hi', 5, 'xy');
	SELECT rpad('hi', 5);
	SELECT rpad('hi', -5, 'xy');
	SELECT rpad('hello', 2);
	--@ SELECT rpad('hi', 5, '');

[codecov]

Codecov Report

Merging #8970 (407ced5) into main (30dca71) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #8970      +/-   ##
==========================================
- Coverage   70.70%   70.70%   -0.01%     
==========================================
  Files        1183     1183              
  Lines      195588   195594       +6     
==========================================
+ Hits       138295   138298       +3     
- Misses      57293    57296       +3     

Flag	Coverage Δ
rust	70.70% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/expr/src/vector_op/string.rs	8.55% <0.00%> (-0.36%)	⬇️

... and 7 files with indirect coverage changes

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