Exclude identities when PERCENTAGE_SPLIT trait is undefined

[emyller]

Users without specified trait appear in segment, evaluated using identity key instead.

Acceptance criteria

• Return false when trait is undefined instead of falling back to identity key

Current implementation:

flagsmith-java-client/src/main/java/com/flagsmith/flagengine/segments/SegmentEvaluator.java

Lines 113 to 117 in 75749e6

	if (contextValue == null) {
	if (context.getIdentity() == null) {
	return false;
	}
	key = context.getIdentity().getKey();

See Flagsmith/flagsmith-engine#283 for context.