feat: add null-safe nullable-argument extensions for querydsl-kotlin

[HarryJhin]

Fixes #1845

Problem

Building dynamic queries in Kotlin is more verbose than in Java. where(...) already ignores
null predicates (DefaultQueryMetadata.addWhere returns early on null), but Kotlin has no
ternary operator, so an optional filter forces if/else, ?.let, a BooleanBuilder, or a
per-field helper. The existing ExpressionExtensions.kt provides only non-null operator overloads,
which do not cover the "skip this condition when the argument is null" case that dominates
dynamic-query code.

Approach

Null-safe infix extensions that return null (dropped by where(...)) when the receiver or the
argument is null:

where(
    member.name eq name,   // name: String? -> skipped when null
    member.age gt age,      // age: Int?      -> skipped when null
)

• New per-type files: SimpleExpressionExtensions, ComparableExpressionExtensions,
  NumberExpressionExtensions, StringExpressionExtensions.
• Each comparison/equality helper has a value form and an expression form
  (gt(value) and gt(expr)), covering parameter and column-to-column comparisons.
• between / notBetween support partial ranges: both bounds yields BETWEEN, one bound yields
  a one-sided comparison, neither is skipped. A ClosedRange overload (age between (20..60)) is
  also provided.
• The existing top-level and / or / xor / xnor / not in ExpressionExtensions.kt are
  folded into null-safe variants (and / or with partial application, the others skip on null).

Compatibility

• Additive: eq / ne / in / gt / goe / lt / loe / between / like / contains
  / startsWith / endsWith reuse member-backed names, so member-over-extension keeps existing
  non-null calls intact.
• Source-breaking: folding and / or / xor / xnor / not changes their return type to
  BooleanExpression?. In practice BooleanExpression.and/or/not stay non-null via their member
  methods, so only raw Expression<Boolean> callers are affected. I'll leave the release and
  version placement entirely to your judgement.

Tests

• The four null combinations (this-null / arg-null / both-null / both-non-null) per function, for
  both the value and expression overloads; partial application for between / notBetween /
  and / or.
• Non-null cases are asserted equal to the member-method result (member-over-extension check).
• Existing CollectionTest still passes. 39 tests total, green with the JDK 17 compile target and
  the JDK 25 build toolchain.

[velo]

Null-safe Kotlin infix extensions are additive, well-documented, and cover all four null combinations in tests. The and/or/xor/xnor/not fold to nullable is an acknowledged minor source-break that stays non-null via the BooleanExpression member methods for typical callers. Full CI matrix green. LGTM.