Add validation test for Kotlin empty when - #2248

Closes #2039

Adds a validation test for empty when.

Kotlin in this module is 2.4.10, i.e. after KT-82844,
so the subject of an empty when is evaluated. The test pins down three distinct cases:

The second row is the non-obvious one: even when the subject has no side effect, the
compiler still emits a single NOP attributed to that line, so the line is reported as
covered rather than empty.

Bytecode of the two subject cases (javap-style dump produced by the validation harness):

  private final emptyWhenWithSideEffectInSubject()V
   L0
    LINENUMBER 27 L0
    ICONST_1
    ISTORE 1
   L1
    LINENUMBER 28 L1      // when (a++) {}
    IINC 1 1
   L2
    LINENUMBER 29 L2
    ILOAD 1
    INVOKESTATIC org/jacoco/core/test/validation/targets/Stubs.nop (I)V

  private final emptyWhenWithoutSideEffectInSubject(I)V
   L0
    LINENUMBER 33 L0      // when (a) {}
    NOP
   L1
    LINENUMBER 34 L1
    ILOAD 1
    INVOKESTATIC org/jacoco/core/test/validation/targets/Stubs.nop (I)V

when {} produces no line number entry at all, hence assertEmpty().

Verification

The naive expectation — that an empty when executes nothing — fails on two of the three
lines, which is what makes the test worth having:

Instructions (KotlinWhenEmptyTarget.kt:24) expected:<[EMPTY]> but was:<[FULLY_COVERED]>
Instructions (KotlinWhenEmptyTarget.kt:29) expected:<[EMPTY]> but was:<[FULLY_COVERED]>

With the assertions as committed: Tests run: 6, Failures: 0.
Full module run for regressions: Tests run: 293, Failures: 0, Errors: 0, Skipped: 0
(mvn -pl org.jacoco.core.test.validation.kotlin test -Djdk.version=21 -Dbytecode.version=21,
Temurin 21.0.11, Maven 3.9.16 — same version as .mvn/wrapper).
spotless:apply leaves both new files unchanged.