Exclude identities when PERCENTAGE_SPLIT trait is undefined

[Zaimwa9]

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:
https://github.com/Flagsmith/flagsmith-rust-flag-engine/blob/main/src/engine_eval/segment_evaluator.rs#L196-L209

See Flagsmith/flagsmith-engine#283 for context.