expression: fix keepNull option for istrue function

[dyzsr]

What problem does this PR solve?

Issue Number: close #17720

Problem Summary:

isTrueOrFalseFunctionClass.keepNull was initialized differently in expression rewriter and EvaluateExprWithNull, which caused outer join wrongly simplified to inner join during predicate push down optimization.

What is changed and how it works?

What's Changed:

• Unify the initialization of isTrueOrFalseFunctionClass with newFunctionImpl.

How it Works:

• Remove the direct initialization of ScalarFunction in wrapWithIsTrue. Invoke NewFunctionInternal instead.
• Use sessionctx.Context to pass the argument is_true_keep_null, which affects the evaluation of NULL in function istrue.

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test

Release note

• fix the behavior of istrue function when null is involved