[fix](Nereids) fix fold constant of string_index in fe

[LiBinfeng-01]

What problem does this PR solve?

Issue Number: close #44666

Related PR: #xxx

Problem Summary:

select string_index('哈哈哈AAA','A', 1); would return whole string when 1 is larger equal than string.splitpart
So we should change splitpart limit to -1 to enable empty character in splitpart list

Release note

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Check List (For Author)

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• Behavior changed:

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• Does this need documentation?

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[LiBinfeng-01]

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[LiBinfeng-01]

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