Fix FlowRuleComparator violating the Comparator contract

[vasiliy-mikhailov]

Problem

FlowRuleComparator.compare returns 0 (equal) for rules that are not actually equal:

if (o1.getLimitApp() == null) {
    return 0;                       // returns 0 even when o2.getLimitApp() is non-null
}
...
} else {
    return 0;                       // two different non-default limitApps compare as equal
}

So a rule whose limitApp is null compares equal to any other rule, and two rules with different non-default limitApp values (e.g. originA vs originB) compare equal. Returning 0 for unequal elements violates the Comparator contract and can cause rules to be dropped when stored in a sorted structure.

Fix

• Return 0 only when both limitApps are null; otherwise order a null consistently against a non-null value.
• Compare two different non-default limitApps with String.compareTo instead of returning 0.

Test

Adds testNullVsNonNullLimitApp and testDifferentNonDefaultLimitApps to FlowRuleComparatorTest, each also checking antisymmetry. Both fail on 1.8 (compare returns 0 for unequal rules) and pass with this change.