[fix](nereids) Fix divide-by-zero guard in divideDecimal constant folding

[LuciferYang]

What problem does this PR solve?

Issue Number: close #64681

Problem Summary:

NumericArithmetic.divideDecimal (the DECIMALV2 constant-folding divide function) guarded against division by zero by checking the numerator (first) instead of the denominator (second):

public static Expression divideDecimal(DecimalLiteral first, DecimalLiteral second) {
    if (first.getValue().compareTo(BigDecimal.ZERO) == 0) {   // wrong operand
        return new NullLiteral(first.getDataType());
    }
    BigDecimal result = first.getValue().divide(second.getValue());
    return new DecimalLiteral(result);
}

As a result, when both operands are DECIMALV2 literals:

• 0 / x folded to NULL instead of 0 — a silent wrong result.
• x / 0 skipped the guard and called BigDecimal.divide(ZERO), throwing ArithmeticException. This is caught by ExpressionEvaluator.invoke and the expression is left unfolded (BE then evaluates it and returns NULL), so it is not a crash, but the FE folding path is incorrect.

The sibling functions divideDouble and divideDecimalV3 both correctly check second. This PR aligns divideDecimal with them by checking the denominator. Division by zero continues to return NULL, matching Doris/MySQL semantics (Doris is MySQL-compatible here; it does not raise the ANSI division by zero error).

Release note

Fix incorrect constant folding of DECIMALV2 division: 0 / x was folded to NULL instead of 0.

Check List (For Author)

• Test

  • Unit Test (added testDivideDecimalZeroNumerator and testDivideDecimalZeroDenominator in NumericArithmeticTest)

• Behavior changed:

  • Yes. 0 / x over constant DECIMALV2 values now correctly folds to 0 instead of NULL. Division by zero still returns NULL (unchanged).

• Does this need documentation?

  • No.

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[yx-keith]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.