Fix RefineSwitchCases generating invalid guard on enum constant labels

[timtebeek]

• Fixes UpgradeToJava25 generates invalid guarded enum switch case #1123

Problem

RefineSwitchCases (part of UpgradeToJava25) converted a case body of the form case X -> { if (cond) {...} } into a guarded case case X when cond -> .... However when guards are only valid on pattern case labels — not on constant labels such as enum constants.

The match guard was labelVariables.containsAll(conditionVariables). For an enum constant label like case OPTION_ONE, extractLabelVariables returns an empty set, and when the if condition referenced no label-bound variables (e.g. checkMethod()), conditionVariables was also empty — so emptySet.containsAll(emptySet) was trivially true. This produced uncompilable code (and a duplicate case OPTION_ONE label):

switch (item) {
    case OPTION_ONE when checkMethod() -> handleOptionOne();
    case OPTION_ONE -> {}
    case OPTION_TWO -> {}
}

Fix

Require the case label to bind at least one pattern variable before refining, so constant/enum/null labels are left untouched.

Testing

Added a regression test (enumConstantLabelWithGuardlessCondition) asserting no change; all existing tests still pass.