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-php-client/src/Engine/Engine.php

Lines 292 to 293 in 9e459f5

	} elseif ($context->identity !== null) {
	$objectIds = [$segmentKey, $context->identity->key];

See Flagsmith/flagsmith-engine#283 for context.